A defibrillator is a medical device that delivers an electric shock to try to restart a heart that has stopped beating properly. When someone's heart suddenly stops or beats in a dangerous, chaotic rhythm, a defibrillator can sometimes save their life by jolting the heart back into a more normal pattern.

You've probably seen defibrillators used in movies or TV shows during medical emergencies: doctors or paramedics place paddles or pads on a patient's chest and call out “Clear!” before delivering the shock. That dramatic warning tells everyone to step back so they don't get shocked too.

Modern public buildings like schools, airports, and shopping malls often have automated external defibrillators (AEDs) mounted on walls in bright cabinets. These machines have simple voice instructions that guide people through the process of helping someone whose heart has stopped. The AED analyzes the person's heart rhythm and decides whether a shock is needed.

Paramedics, emergency room doctors, and other trained people use defibrillators to rescue people in cardiac emergencies.