Coma is a state of unconsciousness brought about by injury to the brain. The injury may be caused by trauma, such as a car accident, or by a lack of oxygen, as in the case of a heart attack or stroke.

Often, people are described as being "comatose" for years on end. However, coma is actually a time-limited state that lasts up to about four weeks after the injury. While in a coma, the person lies motionless with closed eyes. If the person survives the coma without regaining consciousness, he or she enters what is known as a "vegetative state." In this stage, the person can open and close the eyes, stare vacantly, move and make noises, but shows no signs of awareness. About half of those in a vegetative state for one month after trauma will slowly regain awareness.