The Collaborative International Dictionary of English v.0.48:

lunchroom \lunchroom\ n.
   a dining room (in a school, business, or other non-domestic
   facility) where lunch can be purchased.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

lunchroom
    n 1: a restaurant (in a facility) where lunch can be purchased

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "lunchroom":
   automat, beanery, bistro, buffet, buvette, cafe, cafeteria,
   canteen, cantina, chophouse, chuck wagon, coffee, coffee shop,
   coffeehouse, coffeeroom, cookhouse, cookshack, cookshop, diner,
   dining hall, dining room, dog wagon, drive-in, drive-in restaurant,
   eatery, eating house, fast-food chain, grill, grillroom,
   hamburger stand, hash house, hashery, hot-dog stand, kitchen,
   lunch counter, lunch wagon, luncheonette, mess hall, pizzeria,
   quick-lunch counter, restaurant, smorgasbord, snack bar, tavern,
   tearoom, trattoria