The simplest answer is the lack of blood flow to the heart muscle (myocardial infarction). Most of the time this is due to a clot forming in one of the little arteries in your heart, in an area where that artery is already partially blocked off by atherosclerotic plaque (hardening of the arteries due to cholesterol). Occasionally it can be due to spasm of the vessel such as in drug induced heart attacks from cocaine or amphetamines. Heart attack usually refers to myocardial infarction, but lay person might also use it to refer to other heart problems like serious rhythm problems like ventricular fibrillation or ventricular tachycardia.