An electrocardiogram (EKG or ECG) is a quick, noninvasive test that records your heart’s electrical activity. Healthcare providers use it for many reasons, such as to diagnose heart rhythm issues or to monitor how well a treatment is working.
MobiHealthNews: NYT, Bloomberg say Apple is working on Watch EKG, noninvasive glucose
Traditionally, "ECG" usually means a 12-lead ECG taken while lying down as discussed below. However, other devices can record the electrical activity of the heart such as a Holter monitor but also some models of smartwatch are capable of recording an ECG.