The Collaborative International Dictionary of English v.0.48:

Atmosphere \At"mos*phere\, n. [Gr. ? vapor (akin to Skr.
   [=a]tman breath, soul, G. athem breath) + ? sphere: cf. F.
   atmosph[`e]re. See Sphere.]
   1. (Physics)
      (a) The whole mass of a["e]riform fluid surrounding the
          earth; -- applied also to the gaseous envelope of any
          celestial orb, or other body; as, the atmosphere of
          Mars.
      (b) Any gaseous envelope or medium.
          [1913 Webster]

                An atmosphere of cold oxygen.     --Miller.
          [1913 Webster]

   2. A supposed medium around various bodies; as, electrical
      atmosphere, a medium formerly supposed to surround
      electrical bodies. --Franklin.
      [1913 Webster]

   3. The pressure or weight of the air at the sea level, on a
      unit of surface, or about 14.7 lbs. to the sq. inch.
      [1913 Webster]

            Hydrogen was liquefied under a pressure of 650
            atmospheres.                          --Lubbock.
      [1913 Webster]

   4. Any surrounding or pervading influence or condition.
      [1913 Webster]

            The chillest of social atmospheres.   --Hawthorne.
      [1913 Webster]

   5. The portion of air in any locality, or affected by a
      special physical or sanitary condition; as, the atmosphere
      of the room; a moist or noxious atmosphere.
      [1913 Webster] Atmospheric

WordNet (r) 3.0 (2006):

atmosphere
    n 1: a particular environment or surrounding influence; "there
         was an atmosphere of excitement" [syn: atmosphere,
         ambiance, ambience]
    2: a unit of pressure: the pressure that will support a column
       of mercury 760 mm high at sea level and 0 degrees centigrade
       [syn: standard atmosphere, atmosphere, atm, standard
       pressure]
    3: the mass of air surrounding the Earth; "there was great heat
       as the comet entered the atmosphere"; "it was exposed to the
       air" [syn: atmosphere, air]
    4: the weather or climate at some place; "the atmosphere was
       thick with fog" [syn: atmosphere, atmospheric state]
    5: the envelope of gases surrounding any celestial body
    6: a distinctive but intangible quality surrounding a person or
       thing; "an air of mystery"; "the house had a neglected air";
       "an atmosphere of defeat pervaded the candidate's
       headquarters"; "the place had an aura of romance" [syn:
       air, aura, atmosphere]

Moby Thesaurus II by Grady Ward, 1.0:

101 Moby Thesaurus words for "atmosphere":
   action, aerial, aerodynamics, aerosphere, air, ambience, ambient,
   anagnorisis, angle, architectonics, architecture, argument,
   arrangement, atmospheric, aura, background, balance, biosphere,
   brushwork, catastrophe, character, characteristic,
   characterization, climate, color, complication, composition,
   continuity, contrivance, denouement, design, development, device,
   draftsmanship, ecosphere, episode, fable, falling action, feel,
   feeling, flavor, fluid, gas, gaseous envelope, gimmick, grouping,
   halogen gas, impression, incident, individuality, inert gas, lift,
   line, local color, medium, milieu, mise-en-scene, mood, motif,
   movement, mythos, noosphere, note, overtone, painterliness,
   peculiarity, peripeteia, perspective, plan, plot, pneumatic,
   pneumatics, property, quality, recognition, rising action, scheme,
   secondary plot, semblance, sense, shading, shadow, slant, spirit,
   story, structure, subject, subplot, suggestion, surroundings,
   switch, technique, thematic development, theme, tone, topic,
   treatment, twist, undertone, values, welkin