A MID number is a unique identifier that's assigned to a merchant by their acquirer. It's used to identify the merchant within the card network.
The MID number is also sometimes called the merchant account number or the acquirer identification number.
Some acquirers may also have additional numbers that they use in conjunction with the MID number. These are typically called TIDs (terminal ID) or MIDS (merchant ID).
The MID number is used by the card networks to route transactions to the right acquirer. It's also used by the acquirer to identify the merchant and settle transactions.
The MID number is a critical part of the payment process and it's important that merchants keep track of their MID number. If a merchant ever needs to contact their acquirer, they'll need to have the MID number on hand.
