When accepting payment via a credit card or debit card, a merchant is charged a "discount rate" (or "discount fee") by the acquiring bank. For example, when a merchant receives a credit card payment of $100.00, if the bank's discount rate is 3.7%, then the merchant will only actually receive $96.30, with the remaining $3.70 being charged by the bank as per the discount rate. (This answer has a nice summary of how this works.)
My question is, why is the "discount rate" called that? In the most typical usage of the term, a discount is a reduced price charged for something. In the context of the "discount rate" on a merchant receiving a card payment, who is receiving a discount?