n. 1. Any of three standard bipolar limb leads of an electrocardiograph, designated I, II and III. Lead I records potential difference between the right and left arms; lead II, the difference between right arm and left leg; lead III, the difference between left arm and left leg.2.A record obtained from such a lead.ead I records potential difference between the right and left arms; lead II, the difference between right arm and left leg; lead III, the difference between left arm and left leg. 2. A record obtained from such a lead.