A defibrillator is a machine that can help when ventricular fibrillation happens in the heart of a human. A defibrillator can also be named as a life saving device.
Fibrillation happens when many different cells in the heart begin to act as pacemaker cells. This means that many thousands of cells tell the heart to beat, all at different times and with no rhythm. This confusion causes the heart to become unable to move blood around the body. This causes death.
A defibrillator sends a high energy pulse from the top-right of the heart to the bottom-middle of the heart. This causes the whole heart to stop all activity. The heart's normal pacemaker then can try and restart normal beating. Defibrillation does not always work.