gain: A hearing aid's power, measured in the number of decibels that it can add to sound.

gait cycle: The cycle that the feet and legs make when walking, beginning when one heel hits the ground and ending when the same heel hits the ground again.

galactogogue: A substance that promotes breast milk production.

gamma-aminobutyric acid: A chemical messenger in the brain that may help decrease anxiety and promote slow-wave sleep. Often referred to as GABA.

ganglion: An abnormal but harmless mass of tissue, usually nerve cells.

ganglion cyst: A harmless sac of fluid on top of a joint or tendon, usually on the wrist or back of the hand.

gangrene: Death of tissue in part of the body because blood has stopped flowing there.

gastric: Relating to the stomach.

gastritis: Inflammation of the lining of the stomach.

gastroesophageal reflux disease: A condition in which food and acid flow back into the esophagus from the stomach, causing heartburn.

gastrointestinal: Relating to all or some of the organs of the digestive tract, from the mouth to the anus.

gastrointestinal tract: The digestive tract.

gene: Structures on chromosomes that are passed from parent to child. The basic unit of material that passes traits from parent to child.

gene therapy: Correction of a genetic defect by replacing an abnormal gene with a normal gene.

generic drug: A copy of a brand-name drug whose patent has expired. These drugs are less expensive than brand-name drugs.

genetic: Referring to inherited characteristics or genes.

genistein: An antioxidant chemical found in certain plants, like soybeans.

GERD: Abbreviation for gastroesophageal reflux disease, a condition in which food and acid flow back into the esophagus from the stomach, causing heartburn.

geriatric care manager: A trained medical professional who helps families who are caring for older adults.

geriatrician: A physician who specializes in the care of older patients.

gestational diabetes mellitus: A form of diabetes that appears during pregnancy.

GFR: Abbreviation for glomerular filtration rate, the rate at which the kidneys filter excess water and other wastes. A test by the same name is used to determine how well the kidneys are functioning.

giant cell arteritis: Inflammation and damage to blood vessels that supply the head and neck.

gingiva: Another term for gums: a form of oral tissue that covers the roots of teeth and surrounding bone.

gingivitis: Inflammation of the gums.

gland: Any organ or tissue that secretes fluids, such as hormones, for use elsewhere in the body or as waste.

glans penis: The head of the penis.

glaucoma: A condition in which the pressure inside the eye is too high, causing eye damage.

Gleason score: In men with prostate cancer, the Gleason score provides a rough estimate of how fast the cancer is growing.

glenohumeral joint: A shoulder joint that connects the ball of the humerus to the glenoid.

glenoid: The socket of the scapula that connects to the humerus at the shoulder.

glomerular filtration rate: The rate at which the kidneys filter excess water and other wastes. A test by the same name is used to determine how well the kidneys are functioning.

glomeruli: Tiny clusters of capillaries in the kidneys that filter waste products from the blood.

glucagon: A hormone produced in the pancreas that raises blood sugar levels.

glucocorticoids: Steroid hormones released by the adrenal gland when there seems to be a threat, and the stress response is triggered.

glucose: A simple sugar that is the body's main source of energy.

glucose intolerance: Blood sugar levels that are higher than normal, but not yet high enough for a diagnosis of diabetes.

glutamate: A chemical messenger in the brain that may play a role in mood disorders and schizophrenia.

glutamic acid decarboxylase: A protein found in beta cells, the cells that create insulin.

glycemic index: A ranking of foods according to how quickly and how much they raise blood sugar.

glycemic load: A ranking of how much a typical serving of a particular food will raise blood sugar.

glycogen: The body's fuel reserves. This substance, stored primarily in the liver and muscles, is later converted into glucose to provide cells with energy.

glycosylated hemoglobin: The product formed by the attachment of glucose (blood sugar) to hemoglobin (the oxygen-carrying protein in red blood cells). Usually referred to as hemoglobin A1c, or HbA1c.

GnRH: Abbreviation for gonadotropin-releasing hormone, a hormone responsible for the release of follicle-stimulating hormone and luteinizing hormone from the pituitary gland.

GnRH antagonists: Drugs used to treat prostate cancer by blocking the release of luteinizing hormone that do not cause a temporary surge in testosterone.

goblet cells: Cells that produce mucus that line the gastrointestinal tract and lungs.

goiter: An enlarged thyroid gland that creates a lump in the neck.

gonadotropin-releasing hormone: A hormone responsible for the release of follicle-stimulating hormone and luteinizing hormone from the pituitary gland.

gonioscopy: A test used to detect glaucoma that examines the area where fluid drains out of the eye.

gout: A form of arthritis that occurs when uric acid builds up in the joints; The big toe, knee, and ankle joints are most often affected.

graft: Transplanting tissue from one part of the body to another.

gram: A metric unit of weight equivalent to one-thousandth of a kilogram.

Graves' disease: An autoimmune disease that causes the thyroid gland to produce too much thyroid hormone. It is usually accompanied by an enlarged thyroid gland and swollen, bulging, red eyes that appear to stare, as well as occasional double vision and vision loss.

greenstick fracture: A fracture in a young, soft bone in which the bone bends and breaks only on the outer edge of the bend.

grief: A broad range of reactions to bereavement, including sadness, tears, shock, confusion, and anger, among others.

growth factor: A substance produced by the body to stimulate tissue growth.

G spot: The area of sexually sensitive tissue located on the roof of the vagina just inside the opening. Also known as the Grafenberg spot.

guardianship: A legal process of appointing a person to make decisions about important matters (such as health and finances) for someone who is unfit to make those decisions on his or her own.

gullet: The esophagus.

gum disease: Diseases including gingivitis and periodontitis that attack the gum tissue and the structures supporting the teeth. Also called periodontal disease.

gums: A form of oral tissue that covers the roots of teeth and surrounding bone. Also called the gingiva.