The Collaborative International Dictionary of English v.0.48:
Diner \Din"er\, n. One who dines. [1913 Webster]WordNet (r) 3.0 (2006):
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, diner, dining compartment, buffet car] 3: a restaurant that resembles a dining car