A defibrillator is a device that gives a high energy electric shock to the heart through the chest wall to someone who is in cardiac arrest. This high energy shock is called defibrillation, and it's an essential life saving step in the chain of survival. Defibrillation within 3-5 min of collapse can produce survival rates as high as 50-70%. They are specifically designed for the general public to use and there are moves afoot to have them arrive by drone!
Here are some pics of AED's I have seen on my travels around Vlaanderen: