The Collaborative International Dictionary of English v.0.48:

Pampas \Pam"pas\, n. pl. [Sp., fr. Peruv. pampa a field, plain.]
   Vast grass-covered plains in the central and southern part of
   the Argentine Republic in South America. The term is
   sometimes used in a wider sense for the plains east of the
   Andes extending from Bolivia to Southern Patagonia.
   [1913 Webster]

   Pampas cat (Zool.), a South American wild cat (Felis
      pajeros). It has oblique transverse bands of yellow or
      brown. It is about three and a half feet long. Called also
      straw cat.

   Pampas deer (Zool.), a small, reddish-brown, South American
      deer (Cervus campestris syn. Blastocerus campestris).


   Pampas grass (Bot.), a very tall ornamental grass
      (Gynerium argenteum) with a silvery-white silky panicle.
      It is a native of the pampas of South America.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pampas
    n 1: the vast grassy plains of northern Argentina

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "pampas":
   alkali flat, alluvial plain, basin, bottomland, bushveld, campo,
   champaign, champaign country, coastal plain, delta, desert, down,
   downs, fell, flat, flat country, flatland, flats, grass,
   grass veld, grassland, grazing, haugh, haughland, heath, lande,
   lea, level, llano, lowland, lowlands, lunar mare, mare, mead,
   meadow, meadow land, mesa, mesilla, moor, moorland, open country,
   pampa, park, pasturage, pasture, pasture land, peneplain, plain,
   plains, plateau, playa, prairie, range, salt flat, salt marsh,
   salt pan, savanna, sebkha, steppe, steppeland, swale, table,
   tableland, tree veld, tundra, upland, vega, veld, weald,
   wide-open spaces, wold