Clinical death is the popular term for cessation of blood circulation and breathing. It occurs when the heart stops beating in a regular rhythm, a condition called cardiac arrest. The term is also sometimes used in resuscitation research. Stopped blood circulation has historically been difficult to reverse. The absence of blood circulation and vital functions related to blood circulation was considered to be the definition of death. In the middle of the 20th century it became possible to often reverse cardiac arrest through cardiopulmonary resuscitation CPR, defibrillation, epinephrine injection, and other treatments to restore normal heartbeat and circulation. Instead of death, cardiac arrest came to be called "clinical death", meaning the clinical appearance of death. Clinical death is now seen as a medical condition that precedes death rather than actually being dead.