A processing fee, in payments, usually means a payment processing fee: the amount a merchant pays to accept a card, wallet or bank payment. The main entry explains how these fees are calculated and negotiated. In everyday business language the shorter term is often used loosely for the total cost of accepting a payment.
For card payments, that total normally combines three layers: the interchange fee paid to the card issuer, scheme fees charged by the card network, and the margin kept by the acquirer or processor, sometimes shown separately as a processor markup. Under flat-rate pricing the merchant sees one combined figure; under interchange-plus pricing the layers are listed individually.
The term also appears outside merchant acquiring. Lenders may charge a processing fee for arranging a loan, government agencies charge one for handling applications, and some merchants add a processing fee to the customer's bill, which in card payments can count as surcharging and is restricted by network rules and law in some countries.