Merchant is defined in PCI as any entity that accepts Payment Card bearing the logos of any of the five members of PCI SSC (American Express, Discover, JCB, MasterCard or Visa) as payment for goods and/or services.”
Merchant is one member of the The Four-Party Credit Card System
Merchant are Authorized through an Enrollment by a Card Processor (MSP)
Merchants assigned a Merchant Category Code by the Card Issuer when the Merchant first starts accepting Payment Cards