Payment facilitator (payfac)
A company that holds a single contract with an acquirer and then signs up thousands of small merchants underneath it, sparing them the application process.
It used to be that every merchant needed its own acquiring agreement, with credit checks, paperwork and weeks of waiting. A facilitator is formally a "master merchant" that accepts payments on behalf of its sub-merchants and passes the money on. Visa and Mastercard formalised the role in the early 2010s, by which point Square, PayPal and Stripe were already running the model at scale.
The convenience comes at a price for the facilitator. It answers to the acquirer for its sub-merchants' chargebacks and fraud, and it has to do its own know-your-customer checks on every merchant it lets in. Hence accounts that open in five minutes and freeze in five seconds, as plenty of small businesses have discovered the day an unusually large order arrived.