4 KudoZ points awarded for this answer. Comment: "Thanks everyone. I chose مصرف تعاوني for my translation, but I chose this answer because I requested a standard term in my question. This answer fulfilled that criterion and reflected the cooperative character of credit unions. It seems that, at least for US credit unions, there is a gap between standard translations and what the institution does. Another good suggestion is Fuad's term جمعية مصرفية and so is إتحاد مصرفي, but the latter could also mean a syndicate of banks or banking professionals, this same goes for رابطة بنكية. Thanks again to all who helped."