A defibrillator is any device used in the defibrillation of the heart muscle. It is made up of the main unit and is equipped with a set of two electrodes. The electrodes are placed directly on or in the patient. In the case of an internal implanted defibrillator, the entire device is placed inside the body, whereas, with an external defibrillator, defibrillation is carried out externally by placing the electrodes, or "pads", at different points on the torso of the victim or patient.

A defibrillator delivers an electric shock to the heart muscle in an attempt to reintroduce a normal heart rhythm to a someone who has suffered or is suffering cardiac arrest. Cardiac arrest, or sudden cardiac arrest (SCA,) occurs due to "ventricular fibrillation". This is simply a situation in the heart in which an electrical "short-circuit" has occurred, causing the heart muscle to " fibrillate" or quiver. The short-circuiting of the hearts electrical system causes a distortion in the coordinated contractions of the heart muscle causing it to fibrillate