Wiktionary
n. (context legal English) A witness called to give evidence with respect to the reputation, conduct or moral standing of a party.
WordNet
n. a witness who testifies under oath as to the good reputation of another person in the community where that person lives
Usage examples of "character witness".
I made the deposition, after which I suffered uncalled-for abuse from the filthy defence lawyer, who made out that as an informer from a plebeian district I was an unfit character witness.
I'd always been John Reddy's friend--I thought--and I'd to be a character witness for him but, God damn, my parents wouldn't let me.
I just hope Giles has enough sense not to try to go to his rescue by appearing as a character witness, or something.
But her ultimate hope is a mirage: namely, that some farseeing SS officer will take her under his wing, so as to lean on her as a character witness after the German defeat.
As for Shawna, she doesnt have a similar body of written works to cite but, if I may serve as character witness, I can tell you right now she is certainly not a bigot, and a hundred times certainly not an idiot.