The Collaborative International Dictionary of English v.0.48:

Savanna \Sa*van"na\, n. [Of American Indian origin; cf. Sp.
   sabana, F. savane.]
   A tract of level land covered with the vegetable growth
   usually found in a damp soil and warm climate, -- as grass or
   reeds, -- but destitute of trees. [Spelt also savannah.]
   [1913 Webster]

         Savannahs are clear pieces of land without woods.
                                                  --Dampier.
   [1913 Webster]

   Savanna flower (Bot.), a West Indian name for several
      climbing apocyneous plants of the genus Echites.

   Savanna sparrow (Zool.), an American sparrow (Ammodramus
      sandwichensis or Passerculus savanna) of which several
      varieties are found on grassy plains from Alaska to the
      Eastern United States.

   Savanna wattle (Bot.), a name of two West Indian trees of
      the genus Citharexylum.
      [1913 Webster]

WordNet (r) 3.0 (2006):

savanna
    n 1: a flat grassland in tropical or subtropical regions [syn:
         savanna, savannah]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "savanna":
   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, pampas, park, pasturage, pasture, pasture land, peneplain,
   plain, plains, plateau, playa, prairie, range, salt flat,
   salt marsh, salt pan, sebkha, steppe, steppeland, swale, table,
   tableland, tree veld, tundra, upland, vega, veld, weald,
   wide-open spaces, wold



U.S. Gazetteer Places (2000):

Savanna, OK -- U.S. town in Oklahoma
   Population (2000):    730
   Housing Units (2000): 354
   Land area (2000):     1.400910 sq. miles (3.628340 sq. km)
   Water area (2000):    0.009146 sq. miles (0.023688 sq. km)
   Total area (2000):    1.410056 sq. miles (3.652028 sq. km)
   FIPS code:            65600
   Located within:       Oklahoma (OK), FIPS 40
   Location:             34.833229 N, 95.838768 W
   ZIP Codes (1990):
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Savanna, OK
    Savanna


U.S. Gazetteer Places (2000):

Savanna, IL -- U.S. city in Illinois
   Population (2000):    3542
   Housing Units (2000): 1790
   Land area (2000):     2.608884 sq. miles (6.756979 sq. km)
   Water area (2000):    0.093646 sq. miles (0.242541 sq. km)
   Total area (2000):    2.702530 sq. miles (6.999520 sq. km)
   FIPS code:            67821
   Located within:       Illinois (IL), FIPS 17
   Location:             42.090069 N, 90.140050 W
   ZIP Codes (1990):     61074
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Savanna, IL
    Savanna