WordNet (r) 3.0 (2006):

terrain
    n 1: a piece of ground having specific characteristics or
         military potential; "they decided to attack across the
         rocky terrain"

Moby Thesaurus II by Grady Ward, 1.0:

153 Moby Thesaurus words for "terrain":
   Lebensraum, acres, agora, air space, airspace, alluvion, alluvium,
   amphitheater, arable land, area, arena, athletic field, auditorium,
   back country, background, bailiwick, bear garden, belt, bowl,
   boxing ring, bull ring, campus, canvas, champaign, circus, clay,
   clear space, clearance, clearing, clod, cockpit, coliseum,
   colosseum, confines, continental shelf, contour, corridor, country,
   course, crust, demesne, department, desert, dirt, distant prospect,
   district, division, domain, dominion, dry land, dust, earth,
   empty view, environs, field, floor, form, forum, freehold, glade,
   glebe, grassland, ground, gym, gymnasium, hall, heartland,
   hinterland, hippodrome, land, landholdings, landscape, lists,
   lithosphere, living space, locale, marginal land, marketplace,
   marl, mat, milieu, mold, neighborhood, offshore rights,
   open country, open forum, open space, outback, palaestra,
   parade ground, part, parts, pit, place, plain, platform, prairie,
   precinct, precincts, premises, prize ring, profile, province,
   public square, purlieu, purlieus, quarter, range, real estate,
   real property, region, regolith, ring, salient, scene,
   scene of action, scenery, section, setting, shape, site, sod, soil,
   space, sphere, squared circle, stadium, stage, stage set,
   stage setting, steppe, subaerial deposit, subsoil, terra,
   terra firma, territory, the country, theater, three-mile limit,
   tilting ground, tiltyard, topography, topsoil, turf,
   twelve-mile limit, vicinage, vicinity, walk, wide-open spaces,
   wilderness, woodland, wrestling ring, zone