Medically speaking, a coma is the state of protective deep sleep your body goes into when severely injured, as in a car crash. Daytime soap operas would be lost without a good coma every few weeks.
Coma comes from the Greek koma meaning "deep sleep." As with so many words with medical or scientific origins, coma has been appropriated for a far wider range of uses. The zombie-like trance you fall into at family get-togethers like Thanksgiving? That too is a coma, though your chances of recovery are far better than from a real coma. Unless you happen to sit next to Aunt Sadie.