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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WordNet (r) 3.0 (2006):

under the circumstances
    adv 1: because of prevailing conditions; "under the
           circumstances I cannot buy the house"