Cardiac arrest (or cardiopulmonary arrest) occurs when the heart fails to contract effectively to allow circulation of blood through the heart and the rest of the body. During cardiac arrest the heart literally stops beating. When this happens, the body fails to deliver oxygen to vital organs, particularly the brain, leading to cell death. Cardiac arrest is different from a heart attack, in that a heart attack precedes cardiac arrest rather than being a result of it. However, there are many reasons that can lead to cardiac arrest aside from a heart attack. These include stress, direct injury, and neurotoxins that can result in paralysis of the heart muscle.
During surgery by Drs. Toby Russell and Beverly Crusher to replace his crushed spinal cord, Lieutenant Worf went into cardiac arrest, and was clinically dead for a period of time before some backup in his Klingon anatomy restarted his heart. (TNG: "Ethics")
In 2371, Vedek Bareil went into cardiac arrest after suffering third degree burns covering much of his chest, caused by an accident to the vessel carrying the vedek and Kai Winn (among others) to peace treaty negotiations with Cardassia. (DS9: "Life Support")