It is a heart attack
, which means that there is death in the tissue of the heart muscle due to a loss of blood flow to a part of the heart. Coronary Artery
Disease often (but there are other causes) leads to this as the arteries become more and more constricted by the build up of plaque
, until something, such as a clot, completely occludes the blood vessel. At that point you are having an MI.