ST-Elevation Myocardial Infarction (STEMI) is a type of heart attack characterized by a complete blockage of a coronary artery, leading to significant damage to the heart muscle. It is identified by specific changes in the ST segment on an electrocardiogram (ECG). STEMI is a medical emergency requiring immediate treatment to restore blood flow, such as angioplasty, stent placement, or thrombolytic therapy.