Source : Webster's Revised Unabridged Dictionary (1913)

Circumstance \Cir"cum*stance\, 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.

            The circumstances are well known in the country
            where they happened.                  --W. Irving.

   2. An event; a fact; a particular incident.

            The sculptor had in his thoughts the conqueror
            weeping for new worlds, or the like circumstances in
            history.                              --Addison.

   3. Circumlocution; detail. [Obs.]

            So without more circumstance at all I hold it fit
            that we shake hands and part.         --Shak.

   4. pl. Condition in regard to worldly estate; state of
      property; situation; surroundings.

            When men are easy in their circumstances, they are
            naturally enemies to innovations.     --Addison.

   {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}.

Source : WordNet®

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