[1] Historically, it also referred to an instrument to measure atmospheric electrical potential.
An electroencephalogram (EEG) is an electrical recording of the activity of the brain taken from the scalp.
An EEG can be used to diagnose seizures, sleep disorders, and for monitoring of level of anesthesia during surgery.
[2] An electrooculogram (EOG) is an electrical recording of the potential between the cornea and the retina, and does not change with visual stimuli.
Interpretation of an ECG is the basis of a number of cardiac diseases including myocardial infarction (heart attack) and arrhythmias such as atrial fibrillation.