昏迷语出希腊语κῶμα（「沉睡」的意思），是一种深度失去意识的状态。处于这个状态的病人无法被唤醒，对光和声音没有反应，没有睡眠-惊醒周期，也不能随意活动。当脑部受到严重损害或多种原因引起的缺血、缺氧时都可以发生；许多疾病在死亡前可出现昏迷。

昏迷有多种成因，包括中毒、代谢异常、中枢神经系统疾病、严重神经损伤（例如中风）、严重急性感染和糖、水盐代谢、酸碱平衡紊乱等；也有为保护脑部功能而故意施行的昏迷。

昏迷的临床分类：一般按照造成昏迷的解剖位置分成以下三大类： 1.天幕上昏迷（Supratentorial Coma） 2.天幕下昏迷（Subtentorial Coma） 3.弥漫性脑病变昏迷（Diffuse Encephalopathy）

In medicine, coma (from the Greek κῶμα koma, meaning "deep sleep") is a state of unconsciousness in which a person: cannot be awakened; fails to respond normally to painful stimuli, light, or sound; lacks a normal wake-sleep cycle; and does not initiate voluntary actions. A person in a state of coma is described as being comatose. Typically, a distinction is made in the medical community between a coma and a medically induced coma, the former is generally understood to be a result of circumstances beyond the control of the medical community, while the latter is generally understood to be a means by which medical professionals may allow a patient's injuries to heal in a controlled environment.