Interchange fee
The fee an acquirer pays the card issuer on every card purchase, ultimately paid by the merchant.
The card networks set the rates, but the money flows between banks. It is the biggest single component of what a merchant pays to accept cards. Issuers use the revenue for things like reward points and air miles, which means the customer paying cash is effectively subsidising someone else's airport lounge.
Both the EU and the US have legislated on it. The EU's Interchange Fee Regulation 2015/751 capped consumer debit cards at 0.2% of the transaction and credit cards at 0.3%. In the US, the 2010 Durbin Amendment limited debit interchange at large banks to 21 cents plus 0.05%. American credit cards have no cap, which goes a long way towards explaining why American rewards programmes are so generous.