1. Decreased alertness, awareness or consciousness, which may result from a variety of conditions including intoxication (drug, alcohol or toxins), metabolic abnormalities (hypoglycemia, hyperglycemia, ketosis etc.), central nervous system diseases (stroke, head trauma, seizure) and hypoxia. The metabolic abnormalities are the most common causes of coma. The difference between coma and stupor is that a patient with coma can not give a suitable response to either noxious or verbal stimuli, whereas a a patient in a stupor can give a rough response (like screaming) to a noxious stimulus. A persistent coma is called vegetative state.
2. A coma is the tail of a comet produced by vapor boiled off the comet as it nears the sun.