The Collaborative International Dictionary of English v.0.48:

Milieu \Mi`lieu"\ (m[-e]*ly[~e]"), n. [F., fr. mi middle (L.
   medius) + lieu place. See Demi-, Lieu.]
   Environment.

         The intellectual and moral milieu created by multitudes
         of self-centered, cultivated personalities. --J. A.
                                                  Symonds.

         It is one of the great outstanding facts of his
         progressive relation to the elements of his social
         milieu.                                  --J. M.
                                                  Baldwin.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

milieu
    n 1: the environmental condition [syn: milieu, surroundings]

Moby Thesaurus II by Grady Ward, 1.0:

133 Moby Thesaurus words for "milieu":
   agora, air, airspace, alentours, ambiance, ambience, ambient,
   ambit, amphitheater, area, arena, athletic field, atmosphere,
   auditorium, aura, background, bear garden, belt, borderlands, bowl,
   boxing ring, bull ring, campus, canvas, circle, circuit,
   circumambiencies, circumjacencies, circumstances, circus, climate,
   cockpit, coliseum, colosseum, compass, confines, context,
   continental shelf, corridor, country, course, department, district,
   division, element, entourage, environing circumstances,
   environment, environs, feel, feeling, field, floor, forum, gestalt,
   ground, gym, gymnasium, habitat, hall, heartland, hinterland,
   hippodrome, land, lists, locale, marketplace, mat, medium,
   mise-en-scene, neighborhood, note, offshore rights, open forum,
   outposts, outskirts, overtone, palaestra, parade ground, part,
   parts, perimeter, periphery, pit, place, platform, precinct,
   precincts, premises, prize ring, public square, purlieu, purlieus,
   quality, quarter, range, region, ring, salient, scene,
   scene of action, scenery, section, sense, setting, site, situation,
   soil, space, sphere, spirit, squared circle, stadium, stage,
   stage set, stage setting, suburbs, surroundings, terrain,
   territory, theater, three-mile limit, tilting ground, tiltyard,
   tone, total environment, twelve-mile limit, undertone, vicinage,
   vicinity, walk, wrestling ring, zone