Unconsciousness, more appropriately referred to as loss of consciousness or lack of consciousness, is a dramatic alteration of mental state that involves complete or near-complete lack of responsiveness to people and other environmental stimuli. Being in a comatose state or coma is an illustration of unconsciousness. Fainting due to a drop in blood pressure and a decrease of the oxygen supply to the brain is an illustration of a temporary loss of consciousness. Loss of consciousness must not be confused with altered states of consciousness, such as delirium (when the person is confused and only partially responsive to the environment), normal sleep, hypnosis, and other altered states in which the person responds to stimuli.
Loss of consciousness should not be confused with the notion of the psychoanalytic unconscious or cognitive processes (e.g., implicit cognition) that take place outside of awareness.
Loss of consciousness may occur as the result of traumatic brain injury, brain hypoxia (e.g., due to a brain infarction or cardiac arrest), severe poisoning with drugs that depress the activity of the central nervous system (e.g., alcohol and other hypnotic or sedative drugs), severe fatigue, and other causes.
On the other hand, someone who is less than fully conscious cannot give consent to anything. This is relevant in the case of sexual behavior (not allowed with such a person), and also in the case of a patient giving informed consent with regard to starting or stopping a treatment, and euthanasia.
In many countries, First Responders, EMT, or Paramedics must legally obtain consent from a conscious casualty during an emergency before they can assume patient care. If consent is denied, the EMS provider can legally do nothing. In the situation where the casualty is unconscious, in most cases, the EMS provider can assume patient care under the legal assumption of implied consent.