The Collaborative International Dictionary of English v.0.48:

bistro \bistro\ (b[=e]s"tr[=o]; b[i^]s"tr[=o]), n.
   1. a small informal restaurant, especially one serving
      alcoholic beverages.
      [WordNet 1.5]

   2. a nightclub.
      [PJC]

WordNet (r) 3.0 (2006):

bistro
    n 1: a small informal restaurant; serves wine

Moby Thesaurus II by Grady Ward, 1.0:

79 Moby Thesaurus words for "bistro":
   alehouse, automat, bar, barrel house, barroom, beanery,
   beer garden, beer parlor, blind tiger, buffet, buvette, cabaret,
   cafe, cafeteria, canteen, cantina, chophouse, chuck wagon,
   cocktail lounge, coffee shop, coffeehouse, coffeeroom, cookhouse,
   cookshack, cookshop, diner, dining hall, dining room, discotheque,
   dive, dog wagon, dramshop, drinking saloon, drive-in,
   drive-in restaurant, eatery, eating house, fast-food chain,
   gin mill, grill, grillroom, groggery, grogshop, hamburger stand,
   hash house, hashery, honky-tonk, hot-dog stand, kitchen, local,
   lunch counter, lunch wagon, luncheonette, lunchroom, mess hall,
   night spot, nightclub, nitery, pizzeria, pothouse, pub, public,
   public house, quick-lunch counter, rathskeller, restaurant,
   rumshop, saloon, saloon bar, smorgasbord, snack bar, speakeasy,
   supper club, taproom, tavern, tearoom, trattoria, watering place,
   wine shop