The determination and pronouncement of when a person has died has medical, ethical, and legal implications. Laws vary from state to state, and death is usually defined in a state’s statutes.

Definitions and terminology may vary from state to state but generally a person is dead when, according to ordinary standards of medical practice, there is irreversible cessation of the person's spontaneous respiratory and circulatory functions. If artificial means of support preclude a determination that a person's spontaneous respiratory and circulatory functions have ceased, the person is dead when, in the announced opinion of a physician, according to ordinary standards of medical practice, there is irreversible cessation of all spontaneous brain function.

Death occurs when the relevant functions cease. But a formal pronouncement of death is not a legal determination of the cause, manner, or time of death.

In California, the determination of death is governed by the Uniform Determination of Death Act (UDDA), which has been adopted with some variations by most states. According to California Health and Safety Code Section 7180, an individual is considered dead when there is either (1) an irreversible cessation of circulatory and respiratory functions, or (2) an irreversible cessation of all functions of the entire brain, including the brain stem, according to ordinary standards of medical practice. The determination of death must be made by a licensed physician. It's important to note that the pronouncement of death is a medical determination of the fact of death, not a legal determination of the cause, manner, or time of death, which may be subject to further investigation or certification by a coroner or medical examiner.