A customer, consumer, or buyer was an individual or organization who bought products or services from a shop or a business.
In 1962, the Civil Rights Movement was still in its infancy and black people weren't very welcome in Las Vegas, and they could never be customers. (DS9: "Badda-Bing, Badda-Bang")
In 2254, Doctor Phil Boyce had this to say about himself and Captain Christopher Pike, "We both get the same two kinds of customers. The living and the dying." (TOS: "The Cage", "The Menagerie, Part I")