WordNet (r) 3.0 (2006):

flats
    n 1: footwear (shoes or slippers) with no heel (or a very low
         heel)

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "flats":
   alkali flat, alluvial plain, apartment house, basin, bottomland,
   bushveld, campo, champaign, champaign country, coastal plain,
   condominium, cooperative apartment house, delta, desert, down,
   downs, duplex, duplex house, fell, flat, flat country, flatland,
   grass veld, grassland, heath, high-rise apartment building, lande,
   level, llano, lowland, lowlands, lunar mare, mare, mesa, mesilla,
   moor, moorland, open country, pampa, pampas, peneplain, plain,
   plains, plateau, playa, prairie, salt flat, salt marsh, salt pan,
   savanna, sebkha, steppe, table, tableland, tenement, tree veld,
   tundra, upland, vega, veld, weald, wide-open spaces, wold