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.
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            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.
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         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"