Definition
Someone who is both a witness to a crime and an accomplice in that same crime. In some states, a defendant may not be convicted solely on the basis of an accomplice witness' testimony.
Illustrative case law
See, e.g. Cool v. U.S., 409 U.S. 100 (1972) and People v. Bowley, 59 Cal.2d 855 (Cal. 1963).