A "credible witness" is someone who is trustworthy and believable when providing information or giving testimony about something they have experienced or seen. Full definition
We now have so many credible witnesses to such events, we know that this was hardly exceptional. (nextleft.org)
In most civil and criminal cases, a plaintiff is awarded damages or a criminal defendant is convicted or found not guilty based on credible witness testimony. (burnsjainlaw.com)
Indeed, as is evident from what I have quoted from his reasons, he found her to be an entirely credible witness. (vealelaw.com)