sacroiliitis: Inflammation of the sacroiliac joints, which connect the lower spine to the pelvis.

sacrum: The larger triangular bone at the base of the spine.

SAD: Abbreviation for seasonal affective disorder, sadness and depression brought on by a lack of exposure to sunlight. SAD usually appears in the fall or winter and subsides in the spring.

saline: A watery solution that contains a small amount of salt and is often used to administer drugs or as a substitute for plasma.

salivary gland: One of three pairs of glands that pour lubricating fluids and digestive enzymes into the mouth.

saphenous vein: A superficial blood vessel that extends from the thigh to the calf; it can be removed and used as a coronary bypass graft.

sarcolemma: A membrane that covers the muscle fiber and ties the end of it to a tendon.

sarcoma: A cancer that arises in the soft tissues of the body that connect, support, and separate other tissues or organs. Sarcomas can occur almost anywhere in the body.

satisficer: A person who can make a choice and be satisfied with it when presented with an option that meets his or her standards, without needing to examine all options or find the absolute best.

saturated fat: A type of fat found in animal foods such as meat, poultry skin, butter, and whole-milk dairy products, as well in as palm and coconut oils. A diet high in saturated fat tends to raise blood levels of unhealthy low-density lipoprotein (LDL) cholesterol.

sausage digit: A toe or finger swollen and red along its entire length.

scapulothoracic joint: A shoulder joint that connects the scapula to the ribs at the back of the chest.

Schlemm's canal: A circular drainage system in the eye located where the clear cornea, white sclera, and colored iris meet to form an angle.

sciatica: Pain along the course of the sciatic nerve (which runs from the buttock, down the back and side of the leg, and into the foot and toes), often because of a herniated disk.

scintigraphy: A diagnostic technique based on the detection of energy emitted by radioactive substances injected into the body; also called radionuclide scanning.

scintillations: The perception of flashing lights or lines that sometimes occurs during the aura of a migraine headache.

sclera: The white of the eye; a tough, protective coating of collagen and elastic tissue that, with the cornea, makes up the outer layer of the eyeball.

scleral buckling: A surgical technique that indents the sclera and choroid to reattach the retina.

scleroderma: An autoimmune disease in which the skin thickens and hardens; sometimes other parts of the body are affected, and joint pain may result.

scoliosis: An abnormal lateral, or sideways, curvature of the spine.

scotoma: A blank spot in the visual field that is sometimes evident during the aura of a migraine headache.

scurvy: A disease caused by vitamin C deficiency, characterized by bruising, poor wound healing, bleeding of the gums, and loosened teeth.

seasonal affective disorder: Sadness and depression brought on by a lack of exposure to sunlight. Seasonal affective disorder usually appears in the fall or winter and subsides in the spring. sometimes referred to as SAD.

sebaceous gland: A gland that opens into a terminal hair follicle; it secretes sebum, the natural oily conditioner of hair.

seborrheic dermatitis: A mild and common condition that is characterized by an itchy, flaky scalp and that may extend to the ears, face, and chest. Also known as dandruff.

secondary hypertension: High blood pressure that has an identifiable, often correctable, cause such as kidney disease, hyperthyroidism, sleep apnea and other conditions.

secondary osteoporosis: Bone loss associated with an identifiable medical condition, treatment with certain drugs, or immobility.

secretion: The release of chemical substances produced by the body; or the substance that is produced.

sed rate: Shorthand for erythrocyte sedimentation rate—a test involving red blood cells used to check for different infections, inflammations, and cancers.

sedative: A drug or a procedure that has a calming effect and relieves anxiety and tension.

seizure: A sudden, involuntary contraction of muscles that results in rhythmic contortions of the body, often accompanied by a loss of consciousness. Also called a convulsion.

selective estrogen receptor modulators: Chemically synthesized drugs that mimic estrogen in some tissues but act to block estrogen's effects in others.

selective serotonin reuptake inhibitors: Antidepressants that block the reuptake of serotonin into the neurons that released it, leaving more serotonin available to nerve cell receptors. Commonly used to treat depression and other mood disorders.

self-help group: A group of people who meet to discuss and offer assistance to one another with the goal of providing social support for changing troubling behavior patterns.

seminal vesicles: Structures surrounding the prostate gland involved in storing secretions made by the gland.

senescence: Gradual loss of body functions caused by the biological aging process, which increases risk of disease, disability, and death.

senile dementia: Diagnosis once given to people over 65 with dementia.

sensate focus techniques: A set of structured exercises that sex therapists use to help couples focus on the sensual aspects of physical contact without pressure to achieve orgasm.

sensorineural hearing loss: Permanent hearing loss caused by damage to the cochlea, hair cells, or auditory nerve.

sepsis: The destruction or infection of tissues by disease-causing organisms, usually accompanied by a fever.

septicemia: A condition in which disease-causing organisms have spread to the bloodstream from an infection elsewhere in the body. Also known as blood poisoning.

septum: A wall or other structure that divides one cavity from another. For example, in the heart the muscular septum separates the right side of the heart from the left side.

SERMs: Abbreviation for selective estrogen receptor modulators, chemically synthesized drugs that mimic estrogen in some tissues but act to block estrogen's effects in others.

seroma: A pocket of lymphatic fluid that builds up at an incision after surgery.

serotonin: A neurotransmitter that helps regulate sleep and appetite, mediate moods, and inhibit pain.

serotonin-norepinephrine reuptake inhibitors: Antidepressants that slow the reuptake of both serotonin and norepinephrine into the neurons that released these substances, leaving more serotonin and norepinephrine available to nerve cell receptors. Commonly used to treat depression and other mood disorders.

sesamoiditis: A painful inflammation in and around two small bones known as sesamoids, located beneath the base of the big toe, at the ball of the foot.

set: A specific number of repetitions of an exercise done as a group.

sexual dysfunction: A problem with any area of a person's sexual response that causes distress.

shock: A serious medical condition in which there too little blood flows to the outer portions of the body, resulting in cold, sweaty skin; a weak pulse; irregular breathing; and dilated pupils. Shock can be caused by a loss of blood, severe heart problems, severe infections, allergic reactions, or drug overdoses.

short-term memory: Information the brain stores temporarily, from milliseconds to minutes.

shunt: A device inserted into the body to redirect the flow of blood or other fluid from one area to another.

side effect: An unwanted, and sometimes dangerous, reaction caused by medication or other treatment.

sigmoid colon: Section of the colon leading to the rectum that makes an S-shaped curve.

sigmoidoscopy: Internal examination of the rectum and sigmoid colon using a flexible viewing tube inserted through the anus.

signature strengths: Character strengths such as curiosity, integrity, and modesty that people identify with, appreciate having, and enjoy using.

sildenafil citrate: The active ingredient in Viagra. It blocks the breakdown of cyclic guanosine monophosphate, a chemical necessary for an erection.

silent heart attack: Heart attack that occurs without pain or symptoms; occurs most commonly in the elderly or in people with diabetes.

silent ischemia: Shortage of oxygen delivery to the heart muscle that causes no symptoms.

single-photon absorptiometry: A test using gamma rays to measure bone density, usually in the forearm.

sinoatrial node: The natural pacemaker of the heart. Located in the right atrium, the sinoatrial node, sometimes called the sinus node, initiates the heart's electrical activity.

sinus node: A specialized group of heart cells in the right atrium that generate the electrical impulses that cause the heart muscle to contract. Also called the heart's natural pacemaker.

sinus rhythm: The heart's normal rate and rhythm.

skeletal muscles: Muscles attached to bones throughout the body that allow voluntary movement to occur.

skin resurfacing: Any of several approaches to improve skin texture, tone, wrinkle appearance, and discolorations by promoting new collagen and epidermal growth. Chemical peels, dermabrasion, microdermabrasion, and laser procedures are skin-resurfacing techniques.

sleep apnea: Temporary pause in breathing during sleep, lasting at least 10 seconds and associated with a fall in blood oxygen or arousal from sleep. Obstructive sleep apnea is caused by an obstruction in the airway blocking air flow; central sleep apnea occurs when the brain temporarily stops sending signals to the muscles that control breathing.

sleep architecture: The pattern made when sleep stages are charted on a hypnogram.

sleep paralysis: A feeling of paralysis that may occur during the transition between wakefulness and sleep if the REM sleep stage begins before a person is fully asleep; classically associated with narcolepsy.

sleep spindles: On an electroencephalogram (EEG), brief rhythmic bursts of activity that appear during stage 2 sleep.

sling: A slender piece of material surgically inserted under the urethra or bladder neck to provide support and improve continence.

slipped disk: See herniated disk.

slipped vertebra: Forward displacement of a vertebra in relation to the vertebra immediately below; also called spondylolisthesis.

slit lamp: An instrument that magnifies internal structures of the eye with the aid of a thin beam of light. Also called a biomicroscope.

slow-twitch fiber: One of two main types of skeletal muscle fibers. Slow-twitch fibers are recruited most heavily for endurance (aerobic) exercises. See also fast-twitch fiber.

slow-wave sleep: Sleep stages 3 and 4; during slow-wave sleep the brain becomes less responsive to external stimuli.

small intestine: A section of the digestive system that includes the duodenum, jejunum, and ileum and plays the major role in absorbing nutrients for the body.

SMART: An acronym for an approach to setting goals for behavioral change: set a very Specific goal; find a way to Measure progress; make sure it's Achievable; make sure it's Realistic; and set Time commitments.

Snellen chart: The conventional eye chart used to test vision with lines of block letters in progressively smaller sizes.

somatization: Bodily symptoms that have no clear physical cause or are out of proportion to a given ailment, and may stem from psychological causes.

somnambulism: Sleepwalking.

somniloquy: Talking in one's sleep.

sorbitol: A crystalline sugar alcohol used as a sweetening agent.

spacer: A hollow chamber into which inhaled medicines can be squirted before inhalation. Spacers are used with metered-dose inhalers to help deliver medicine effectively to the bronchial tubes and to reduce the amount of medicine left behind on the tongue and throat.

spasm: An involuntary muscle contraction.

sphincter: A ring of muscle that surrounds an opening and can be contracted to close the opening. For example, the muscles found at the anus and the opening of the bladder are sphincters.

sphygmomanometer: A device for measuring blood pressure.

spina bifida: A congenital defect in which part of the spinal column fails to develop completely, leaving part of the spinal cord exposed.

spinal fusion: A procedure to attach two or more vertebrae with a bone graft in order to eliminate motion and relieve pain.

spinal stenosis: A narrowing of the spinal canal, which can result in compression of nerve roots.

spinal tap: Use of a hollow needle to withdraw fluid from the lower part of the spinal canal for testing. Also called a lumbar puncture.

spinous process: The lever-like backward projection extending off each vertebra, to which muscles and ligaments are attached.

spirometer: A device that measures airway obstruction, used to diagnose asthma and determine the severity of the condition.

spirometry: A simple, painless breathing test performed in a physician's office or pulmonary function laboratory that measures how fast air can be forced from the lungs and the total amount of air that can be emptied from the lungs.

splenic flexure syndrome: A painful spasm in the left upper abdomen below the rib cage, produced by areas of trapped gas in the colon.

spondylolisthesis: Forward displacement of a vertebra in relation to the vertebra immediately below.

spondylosis: A general term for degeneration of the spine that causes narrowing of the spinal canal and the small openings (intervertebral foramina) through which spinal nerves exit the canal.

spongy bone: Porous bone, also called trabecular bone, often found at the center of long bones.

sprain: A stretched or torn ligament.

sputum: A mixture of saliva and mucus that is coughed up from the respiratory tract. Sputum may be examined in a laboratory for signs of disease.

squamous cell: Flat, scaly epithelial cell.

SSRIs: Abbreviation for selective serotonin reuptake inhibitors, antidepressants that block the reuptake of serotonin into the neurons that released it, leaving more serotonin available to nerve cell receptors. Commonly used to treat depression and other mood disorders.

stable angina: Angina pectoris (chest pain with exertion or stress) that is well-controlled with medicines and lifestyle changes.

stable coronary artery disease: Narrowings in the heart arteries that cause angina pectoris in a predictable and stable pattern over time (for example, after walking a certain distance).

stages of change: A model for how people make changes in their lives. According to this model, changes in behavior are made gradually and in relatively distinct stages.

staging: The process of determining how far cancer has progressed. Staging is often used to determine the best course of treatment.

standardized extract: An herbal product in which what is believed to be the active ingredient meets an established standard of strength.

statins: Cholesterol-lowering medications that interfere with the enzyme 3-hydroxy-3-methylglutaryl-coenzyme A reductase; also known as HMG-CoA reductase inhibitors. Statins work by changing the way the liver processes lipids.

stenosis: An abnormal narrowing of a passageway, such as a blood vessel, or other type of opening in the body.

stent: A wire mesh device inserted into an artery to prop it open once a blockage has been cleared by angioplasty.

sterilization: 1) A surgical procedure or other method that results in a person being unable to reproduce. 2) The process by which materials are thoroughly cleaned of all organisms that could cause disease or infection.

steroids: Another term for corticosteroids—steroid medications made to mimic hormones produced naturally by the adrenal glands. They are used to treat a wide range of health problems.

stimulant: A substance that speeds up chemical reactions inside cells and provides a boost of energy. Examples include caffeine and amphetamine.

stomach: The sac-like organ of the digestive system between the esophagus and the duodenum which breaks down food and moves it along to the small intestine to be digested.

strain: A stretched or torn muscle or tendon, usually caused by accident, misuse, or overuse.

stratum corneum: The most superficial layer of the epidermis.

strength: The ability of muscles to exert force.

strength training: Popular term for exercises that harness resistance supplied by body weight, free weights such as dumbbells or weighted cuffs, resistance bands, or specialized machines; also known as resistance training or weight training.

streptokinase: A thrombolytic (clot-dissolving) agent designed to dissolve the blood clots that block an artery during a heart attack or stroke.

stress: An innate survival response in which certain hormones are released, increasing blood flow to the brain or heart. The stress response leads to an energy surge, enabling a person to flee dangerous situations. Ongoing stress, however, can sap energy and damage health.

stress fracture: A hairline crack in a bone that usually occurs from overuse; left untreated, this may lead to displacement of the bones.

stress response: Physiological changes, such as quickened breathing and heartbeat and increased blood pressure, brought on by stress hormones released in response to a real or perceived threat to safety. Also called the fight-or-flight response.

stress test: A diagnostic test in which cardiovascular measurements such as heart rate, blood pressure, and electrical activity are recorded while the heart is being stressed (usually by having the person exercise on a treadmill or bicycle).

stressors: Stressful events or circumstances that may be real or perceived threats to equilibrium and well-being.

stria: A line, streak, or band, such as the stretch marks that occur in pregnancy.

stricture: The abnormal narrowing of a hollow passage in the body, such as the esophagus or the urethra.

stroke: Blockage or rupture of a blood vessel supplying the brain; often leads to impaired brain function or death.

stupor: A state of lethargy and unresponsiveness.

subacute: A disease or condition that progresses slower than an acute condition but faster than a chronic condition.

subacute thyroiditis: A painful version of thyroid inflammation caused by viral infection. Symptoms are flu-like and include fever, muscle aches and pains, and a painful, swollen thyroid gland. Also known as de Quervain's thyroiditis.

subarachnoid hemorrhage: A hemorrhagic stroke that occurs when a blood vessel on the surface of the brain bursts and bleeds into the space between the brain and the skull; usually caused by an aneurysm or other blood vessel malformation.

subarachnoid space: Space inside the brain where cerebrospinal fluid circulates.

subcutaneous: Beneath the skin.

subcutaneous tissue: Deepest layer of skin, which consists of connective tissue and fat.

subdural hematoma: A blood clot in the brain between the cerebral cortex and the dura.

subendocardial myocytes: Heart-muscle cells on the inside of the heart chambers; these cells are highly susceptible to damage from blockages of the major coronary arteries.

substance abuse: Continued substance use despite substance-related social or interpersonal problems.

substance dependence: A condition characterized by excessive and often compulsive substance use, impaired control over substance use, continued use of substances despite adverse consequences, and withdrawal symptoms that emerge when the substance use is discontinued.

subunit vaccines: Vaccines using only part of a microbe—the antigens—to elicit an immune response; these vaccines tend to cause fewer adverse reactions than vaccines which contain the whole microbe.

sulcus: The V-shaped hollow at the margin of the tooth and gum.

sulfonylureas: A class of medications that works by stimulating the pancreas to produce more insulin.

sundowning: Confusion or disorientation beginning at the end of the day and continuing into the night; often occurs in people with Alzheimer's disease or vascular dementia.

superior vena cava: The major vein that carries deoxygenated blood from the upper body to the heart.

superset: Two or more exercises combined for a more vigorous workout. During a superset, all the reps of exercise A are performed and then all the reps of exercise B before resting.

suppository: A solid form of medication that is inserted in the rectum or vagina and absorbed into the bloodstream.

suprachiasmatic nucleus: A small group of nerve cells located in the hypothalamus that controls the sleep/wake cycle.

supraventricular tachycardia: An abnormally fast heartbeat originating in heart tissue above the ventricles.

suture: The process of sewing tissues together after surgery; or the stitch itself.

sympathetic nervous system: An offshoot of the autonomic nervous system; it sends signals to prepare the body for action when stress hormones are released in response to perceived or real dangers.

symptom-limited exercise stress test: Exercise test, usually using a treadmill or bicycle, that increases in difficulty at set stages and is stopped when the person develops chest pain, breathlessness, or extreme fatigue.

synapse: The junction between two neurons, across which chemical neurotransmitters carry messages.

syncope: Fainting or loss of consciousness caused by a temporary shortage of oxygen in the brain.

synovectomy: Surgical removal of the synovial membrane that lines the joints.

synovial fluid: A thick liquid that lubricates the joints and tendons.

synovial joint: The most mobile type of joint; found in the shoulders, wrists, fingers, hips, etc.

synovitis: Inflammation of the synovium.

synovium: A thin membrane that lines joint capsules and produces synovial fluid.

systemic: Pertaining to something that affects the whole body rather than separate organs or parts.

systemic lupus erythematosus: A connective tissue disease that can affect internal organs, nervous system, skin, and joints.

systole: The brief period during which the heart contracts during a normal heartbeat, pumping blood into the aorta and the pulmonary artery.

systolic blood pressure: The first or top number in a blood pressure reading; a measure of the pressure blood exerts against arterial walls when the heart contracts.

systolic heart failure: The inability of the heart to pump blood efficiently due to weakening and enlargement of the ventricles. Systolic heart failure is usually caused by coronary artery disease, high blood pressure, and valvular heart disease.