The Collaborative International Dictionary of English v.0.48:

Plateau \Pla*teau"\, n.; pl. F. Plateaux (F. ?; E. ?), E.
   Plateaus. [F., fr. OF. platel, properly a little plate. See
   Plate.]
   1. A flat surface; especially, a broad, level, elevated area
      of land; a table-land.
      [1913 Webster]

   2. An ornamental dish for the table; a tray or salver.
      [1913 Webster]

WordNet (r) 3.0 (2006):

plateau
    n 1: a relatively flat highland [syn: tableland, plateau]

Moby Thesaurus II by Grady Ward, 1.0:

122 Moby Thesaurus words for "plateau":
   alkali flat, alluvial plain, amount, basin, bench, bottomland,
   break, bushveld, caliber, campo, champaign, champaign country,
   coastal plain, coffee break, compass, cut, degree, delta, desert,
   down, downs, downtime, extent, fell, flat, flat country, flatland,
   flats, grade, grass veld, grassland, half time,
   half-time intermission, heath, height, highland, holiday, interim,
   interlude, intermezzo, intermission, intermittence, interregnum,
   interruption, interval, lande, leap, letup, level, llano, lowland,
   lowlands, lull, lunar mare, mare, mark, measure, mesa, mesilla,
   moor, moorland, notch, nuance, off-time, open country, pampa,
   pampas, pas, pause, peg, peneplain, period, pitch, plain, plains,
   plane, playa, point, point of repose, prairie, proportion,
   quiet spell, range, ratio, reach, recess, relief, remove, respite,
   resting point, round, rung, salt flat, salt marsh, salt pan,
   savanna, scale, scope, sebkha, shade, shadow, space, stair,
   standard, step, steppe, stint, table, table mountain, tableland,
   time off, time out, tread, tree veld, tundra, upland, vacation,
   vega, veld, weald, wide-open spaces, wold