Definition: A credit card issuer is a bank or credit union who offers credit cards. The credit card issuer makes the credit limit available to cardholders and is responsible for sending payments to merchants for purchases made with credit cards from that bank. Credit card issuers are also called member banks. Chase and Citi are examples of credit card issuers.
Credit card issuers can't issue credit cards all by themselves, they need the help of payment processing networks like Visa and MasterCard. However, American Express and Discover act as both the credit card issuers and the payment processing network.