What does steme mean in medical terminology?

I think you refer to a STEMI. This stands for an ST-Elevation Myocardial Infarction.

This is where a thrombus (clot) forms, usually due to a fat plaque (atheroma) cap breaking and a clot forming over it in one of the coronary arteries that supplies blood to the heart muscle, causing a heart attack.

The term ST elevation refers to a characteristic waveform seen on ECG/EKG in these incidents.