An AED is an acronym for Automated External Defibrillator.  An automated external defibrillator or AED is a portable electronic device that automatically diagnoses the potentially life threatening cardiac arrhythmias of the heart and is able to treat them through defibrillation, the application of electrical therapy which stops the arrhythmia, allowing the heart to reestablish an effective rhythm. Such as in the case of Sudden Cardiac Arrest.