recording of the magnitude of the heart's electrical potential measured from twelve different angles ("leads") over a period of time (usually ten seconds)

An electrocardiogram (ECG or EKG) is a record of the electrical activity of the heart over a period of time. The instrument that makes the record is the ECG machine or Electrocardiograph. Willem Einthoven invented it. It works by attaching electrodes to the outer surface of the skin.

A normal ECG

An electrocardiogram monitors the heart. Each beat of the heart is triggered by an electrical impulse, normally generated from special cells in the upper right chamber of the heart. An electrocardiogram records these electrical signals as they travel through the heart. Doctors can use an electrocardiogram to look for patterns among these heartbeats and rhythms to diagnose various heart conditions.

ECG is used to measure the rate and regularity of heartbeats, the size and position of the chambers, the presence of any damage to the heart, and the effects of drugs or devices used to regulate the heart, such as an artificial pacemaker.

An electrocardiogram is a painless test. The results of your electrocardiogram will likely be reported immediately after it is done.