MID: A merchant identification number (MID) is a unique number assigned to a merchant account to identify it throughout the course of processing activities.
When merchants process credit-card transactions over a computer network, those networks are interconnected with a credit card front-end processing center and a back-end settlement network via a network gateway. The merchant account comprises a merchant identification number (MID), one or more terminal identification numbers (TID), and a gateway identification number (GID).
In particular, the MID uniquely identifies the merchant, while the TID specifies a particular profit center, location or facility from which credit-card transactions are being processed. The GID identifies the particular network gateway through which credit card transactions from the merchant will be routed (to the front-end processing center and back-end settlement network). The MID specifies the merchant, but all of the identifiers taken together will confirm each facet of the transaction for greater security.
