Cardiac arrest (or cardiopulmonary arrest) occurred when the heart failed to contract effectively to allow circulation of blood through the heart and the rest of the body. During cardiac arrest the heart literally stopped beating. When this happened, the body failed to deliver oxygen to vital organs, particularly the brain, leading to cell death. Cardiac arrest was different from a heart attack, in that a heart attack precedes cardiac arrest rather than being a result of it. However, there were many reasons that could 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")