The Collaborative International Dictionary of English v.0.48:
Corporal \Cor"po*ral\ (k[^o]r"p[-o]*ral), n. [Corrupted fr. F.
caporal, It. caporale, fr. capo head, chief, L. caput. See
Chief, and cf. Caporal.] (Mil.)
A noncommissioned officer, next below a sergeant. In the
United States army he is the lowest noncommissioned officer
in a company of infantry. He places and relieves sentinels.
Corporal's guard, a detachment such as would be in charge
of a corporal for guard duty, etc.; hence, derisively, a
very small number of persons.
Lance corporal, an assistant corporal on private's pay.
Ship's corporal (Naut.), a petty officer who assists the
master at arms in his various duties.