According to the American Heart Association: "A heart attack occurs when the blood supply to part of the heart muscle itself -- the myocardium -- is severely reduced or stopped. The medical term for heart attack is myocardial infarction. The reduction or stoppage happens when one or more of the coronary arteries supplying blood to the heart muscle is blocked. This is usually caused by the buildup of plaque (deposits of fat-like substances), a process called atherosclerosis. The plaque can eventually burst, tear or rupture, creating a "snag" where a blood clot forms and blocks the artery. This leads to a heart attack. A heart attack is also sometimes called a coronary thrombosis or coronary occlusion."
Some heart attacks are sudden and intense -- the "movie heart attack," where no one doubts what's happening. But most heart attacks start slowly, with mild pain or discomfort. Often people affected aren't sure what's wrong and wait too long before getting help. The American Heart Association lists the following signs that can mean a heart attack is happening: