The Collaborative International Dictionary of English v.0.48:
Codefendant \Co`de*fend"ant\, n. A joint defendant. --Blackstone. [1913 Webster]WordNet (r) 3.0 (2006):
co-defendant n 1: a defendant who has been joined together with one or more other defendants in a single action [syn: codefendant, co-defendant]Bouvier's Law Dictionary, Revised 6th Ed (1856):
CO-DEFENDANT. One who is made defendant in an action with another person.