A coma is a state of prolonged unconsciousness where a person cannot be awakened and fails to respond to stimuli. It can result from various causes, including traumatic brain injury, stroke, or drug overdose. During a coma, the brain's functions are severely impaired, and the individual shows no signs of awareness or purposeful behavior. Related states include vegetative state and minimally conscious state. In a vegetative state, a person may have sleep-wake cycles but lacks awareness of their surroundings. A minimally conscious state involves inconsistent but discernible signs of awareness, such as following simple commands or responding to stimuli.