The Collaborative International Dictionary of English v.0.48:

circumstance \cir"cum*stance\ (s[~e]r"k[u^]m*st[a^]ns), n. [L.
   circumstantia, fr. circumstans, -antis, p. pr. of circumstare
   to stand around; circum + stare to stand. See Stand.]
   1. That which attends, or relates to, or in some way affects,
      a fact or event; an attendant thing or state of things.
      [1913 Webster]

            The circumstances are well known in the country
            where they happened.                  --W. Irving.
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   2. An event; a fact; a particular incident.
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            The sculptor had in his thoughts the conqueror
            weeping for new worlds, or the like circumstances in
            history.                              --Addison.
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   3. Circumlocution; detail. [Obs.]
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            So without more circumstance at all
            I hold it fit that we shake hands and part. --Shak.
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   4. pl. Condition in regard to worldly estate; state of
      property; situation; surroundings.
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            When men are easy in their circumstances, they are
            naturally enemies to innovations.     --Addison.
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   Not a circumstance, of no account. [Colloq.]

   Under the circumstances, taking all things into
      consideration.

   Syn: Event; occurrence; incident; situation; condition;
        position; fact; detail; item. See Event.
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The Collaborative International Dictionary of English v.0.48:

Circumstance \Cir"cum*stance\, v. t.
   To place in a particular situation; to supply relative
   incidents.
   [1913 Webster]

         The poet took the matters of fact as they came down to
         him and circumstanced them, after his own manner.
                                                  --Addison.
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WordNet (r) 3.0 (2006):

circumstance
    n 1: a condition that accompanies or influences some event or
         activity
    2: the set of facts or circumstances that surround a situation
       or event; "the historical context" [syn: context,
       circumstance, setting]
    3: information that should be kept in mind when making a
       decision; "another consideration is the time it would take"
       [syn: circumstance, condition, consideration]
    4: formal ceremony about important occasions; "pomp and
       circumstance"

Moby Thesaurus II by Grady Ward, 1.0:

115 Moby Thesaurus words for "circumstance":
   absolute fact, accepted fact, actual fact, adjunct, admitted fact,
   adventure, affair, appurtenance, aspect, axiom, bald fact,
   bare fact, bearings, brutal fact, case, circumstances, cold fact,
   component, conceded fact, condition, conditions, constituent,
   contents, datum, demonstrable fact, destiny, detail, doom, element,
   empirical fact, episode, established fact, estate, event,
   experience, fact, fact of experience, factor, feature, finances,
   fix, fixings, footing, formality, given fact, go, hap, happening,
   happenstance, hard fact, heraldry, incident, income,
   indisputable fact, inescapable fact, ingredient, integrant, item,
   jam, kismet, location, lot, makings, matter of fact, modality,
   mode, moira, naked fact, not guesswork, not opinion, occasion,
   occurrence, part, part and parcel, particular, pass, phenomenon,
   pickle, place, plain, plight, pomp, portion, position,
   positive fact, postulate, posture, predicament, pride,
   provable fact, rank, reality, resources, salient fact,
   self-evident fact, significant fact, simple fact, situation,
   sober fact, solemnity, specialty, spot, standing, state,
   state of affairs, station, status, stubborn fact, the case,
   the nitty-gritty, thing, turn of events, undeniable fact, weird,
   well-known fact