The medical abbreviation MI stands for Myocardial Infarction, which is also commonly known as a heart attack. A heart attack occurs when there is a sudden blockage of blood flow to the heart, which results in damage to the heart muscle. The blockage is usually caused by a blood clot forming in one of the coronary arteries that supply blood to the heart. MI is a serious medical emergency that requires immediate treatment to prevent further damage to the heart and other complications. Symptoms of MI may include chest pain or discomfort, shortness of breath, sweating, nausea, and lightheadedness.