Automated external defibrillator: A device that automatically analyzes the heart rhythm and, if it detects a problem that may respond to an electrical shock, that permits a shock to be delivered to restore a normal heart rhythm. Abbreviated AED. Thanks to their small size and ease of use, AEDs have been installed in many settings (such as schools and airports) and serve a role in expanding the number of opportunities for life-saving defibrillation.