Plaque is a buildup of cholesterol, white blood cells, calcium, and other substances in the walls of arteries. Over time, plaque narrows the artery, and the artery hardens (atherosclerosis).

Plaque sometimes reduces blood flow to the heart muscle, which can cause chest pain (angina). Plaque in the large artery in the neck (carotid artery stenosis) may block blood flow to the brain and is a common cause of transient ischemic attack (sometimes called "mini-stroke") and stroke.

Plaques are covered with a fibrous cap, which may rupture if some trigger causes a surge in blood pressure or causes the artery to constrict. A person may have a heart attack if a plaque breaks open, creating a blood clot that completely blocks blood flow through the artery.