A small battery-operated electronic device that is surgically implanted under
the skin and joined to the heart by wires, and that measures the pulse and
corrects too fast or too slow heart rhythms.
An ICD [implantable cardiac defibrillator] is a pacemaker-like device
implanted under the skin. Wires called "leads" are placed in the heart to
monitor the heart rate. When the device detects a potentially deadly heart
rhythm disorder (arrhythmia), the ICD delivers a controlled, electric shock to
restore the heart's normal rhythm.
Source: Heart Rhythm Society Heart
Arrhythmia - Circulatory system - Ventricular fibrillation