Source : Webster's Revised Unabridged Dictionary (1913)

Diner \Din"er\, n.
   One who dines.

Source : WordNet®

diner
     n 1: a person eating a meal (especially in a restaurant)
     2: a passenger car where food is served in transit [syn: {dining
        car}, {dining compartment}, {buffet car}]
     3: a restaurant that resembles a dining car