The Collaborative International Dictionary of English v.0.48:

Soiree \Soi`ree"\, n. [F., fr. soir evening, fr. L. serus late,
   serum late time. Cf. Serenade.]
   An evening party; -- distinguished from levee, and
   matin['e]e.
   [1913 Webster]

WordNet (r) 3.0 (2006):

soiree
    n 1: a party of people assembled in the evening (usually at a
         private house)

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "soiree":
   affair, assemblee, assembly, assignation, at home, ball, brawl,
   caucus, colloquium, commission, committee, conclave, concourse,
   congregation, congress, conventicle, convention, convocation,
   council, dance, date, diet, eisteddfod, evening, festivity, fete,
   forgathering, forum, gathering, get-together, housewarming, levee,
   matinee, meet, meeting, panel, party, plenum, prom, quorum, rally,
   reception, rendezvous, reunion, salon, seance, session, shindig,
   sit-in, sitting, sociable, social, social affair, social gathering,
   symposium, synod, turnout, wake