A defibrillator is a device that gives a high energy electric shock to the heart of someone who is in cardiac arrest. This high energy shock is called defibrillation, and it's an essential part in trying to save the life of someone whos in cardiac arrest. A defibrillator may also be called a defib, an AED (Automated External Defibrillator) or a PAD (Public Access Defibrillator).