A coma is a deep state of unconsciousness, characterized by an inability to respond to stimuli. There are grades or levels of coma corresponding to an afflicted individual's ability to respond to stimuli. A complete or irreversible coma is usually characterized by an inability to respond to stimuli or, conversely, a mild coma is often characterized by a less complete sensory unreceptivity in patients.

In addition to coma, there are other stages of consciousness including lethargy and stupor that reflect a greater degree of brain activity and awareness. With some injuries and disease processes, it is possible for patient to move between varying states of consciousness, excepting irreversible coma. Accordingly, the ability to define and determine irreversible coma and brain death are important considerations in medical ethics and in the practical treatment of patients.

In addition to an inability to respond to stimuli, there are a number of indications of coma. With irreversible coma, there is a changed electroencephalogram (i.e., an EEG that shows a lack of electrical activity found in the normally functioning brain). Patients in a comatose state are unable speak or respond to normally painful stimuli such as pinpricks. In many cases, there is also an inability to breath without mechanical assistance.

Coma can be induced by a number of factors, most commonly traumatic brain injury. Diabetic coma may also result from a catastrophic acidosis induced by diabetes mellitus. Hepatic comas are those comas brought on by damage to cerebral cells resulting from metabolic defects produced by damage or degeneration (cirrhosis) of hepatic (liver) cells. Acute alcohol or drug intoxication may also lead to coma.