What does the medical abbreviation ST elevation MI mean?

This stands for an ST segment elevated, myocardial infarction. This is a subtype of myocardial infarcation aka a heart attack whereby the heart does not receive sufficient oxygen, commonly due to a blockage of one of the coronary (heart) arteries leading to heart muscle death.

It is named so because on an ECG/EKG, you can characteristically see an elevation developing in the ST segment i.e. the end of electrical depolarisation and ventricular repolarisation over time.

This is contrasted with a non-ST elevated MI aka an NSTEMI, also known as unstable angina, that does not show this classical ST elevation.