STEMI is an acronym meaning "ST segment elevation myocardial infarction," which is a type of heart attack. This is determined by an electrocardiogram (ECG) test.
Myocardial infarctions (heart attacks) occur when a coronary artery suddenly becomes at least partially blocked by a blood clot, causing at least some of the heart muscle being supplied by that artery to become infarcted (that is, to die). Heart attacks are divided into two types, according to their severity. A STEMI is the more severe type.
In a STEMI, the coronary artery is completely blocked off by the blood clot, and as a result virtually all the heart muscle being supplied by the affected artery starts to die.
This more severe type of heart attack is usually recognized by characteristic changes it produces on the ECG. One of those ECG changes is a characteristic elevation in what is called the "ST segment." The elevated ST segment indicates that a relatively large amount of heart muscle damage is occurring (because the coronary artery is totally occluded), and is what gives this type of heart attack its name.
Click to read read about the "milder" form of heart attack - the NSTEMI.