A person is considered to be dying when they are in the process of passing from life to death.
It is characterized by a gradual failing of vital functions — including the cardiac, respiratory, and central nervous systems — followed by an absence of criteria that define life (spontaneous heartbeat, breathing, and brain function).
The dying phase is generally considered to be minutes to hours in duration, but can occasionally be just seconds. It may also be days in duration, where the loss of function indicates that death is imminent.

Ministry of Health. 2015. New Zealand Palliative Care Glossary. Wellington: Ministry of Health.