Merchant account
The agreement with an acquirer that lets a business accept card payments, and the account where the money sits before it is paid out.
Before a shop can take its first card, the acquirer vets it much as a bank vets a borrower. The reason is chargebacks: if the shop goes bust with a hundred undelivered orders, the acquirer is the one refunding the cardholders. Underwriting looks at the line of business and merchant category code, expected volume, delivery times and past history. What comes out is a merchant ID (MID), a pricing agreement and often a reserve held back for a while.
For decades this meant paperwork and weeks of waiting. Stripe and Square built businesses on letting small firms skip it by becoming sub-merchants of a payment facilitator instead. Larger merchants still get their own account, because the fees are lower and an algorithm can't freeze it on a Friday evening.