diner

Etymology

dine + -er. Doublet of dinner.

noun

1. One who dines.
   The most rapid and most seductive transition in all human nature is that which attends the palliation of a ravenous appetite.[…]Can those harmless but refined fellow-diners be the selfish cads whose gluttony and personal appearance so raised your contemptuous wrath on your arrival? 1922, Ben Travers, chapter 5, in A Cuckoo in the Nest
   When it comes to Chinese food I have always operated under the policy that the less known about the preparation the better. A wise diner who is invited to visit the kitchen replies by saying, as politely as possible, that he has a pressing engagement elsewhere. 1983, Calvin Trillin, Third Helpings
2. A dining car in a railroad train.
   The diner is everybody's kitchen. 1979, Richard Gutman, American Diner
3. (US) A typically small restaurant, usually modeled after a railroad dining car, that serves lower-class fare, normally having a counter with stools along one side and booths on the other, and often decorated in 50s and 60s pop culture themes and playing popular music from those decades.