A merchant acquirer is the bank or licensed payment institution that contracts with a business to accept card payments, and the role is described in full under acquirer. The word "merchant" is added to make clear which side of the card transaction is meant, since in other areas of finance "acquirer" can mean a company that buys another company.
The merchant acquirer provides the merchant account, routes authorisation requests to the card networks, submits transactions for clearing and pays the merchant the proceeds, minus the merchant discount rate. It also underwrites merchants, monitors their fraud and chargeback levels, and bears the loss if a merchant fails while owing money to cardholders. People also call it an acquiring bank, although many acquirers today are non-bank payment institutions rather than banks.
In everyday use, "merchant acquirer" is sometimes applied loosely to payment service providers and payment facilitators that sell card acceptance to businesses. Strictly, those firms usually work through a licensed acquirer that holds the card network membership.