The Collaborative International Dictionary of English v.0.48:

S'eance \S['e]`ance"\, n. [F., fr. L. sedens, -entis, p. pr. of
   sedere to sit. See Sit.]
   A session, as of some public body; especially, a meeting of
   spiritualists to receive spirit communications, so called.
   [1913 Webster]

WordNet (r) 3.0 (2006):

seance
    n 1: a meeting of spiritualists; "the seance was held in the
         medium's parlor" [syn: seance, sitting, session]

Moby Thesaurus II by Grady Ward, 1.0:

80 Moby Thesaurus words for "seance":
   assemblee, assembly, assignation, at home, audience, ball,
   bargaining, bargaining session, brawl, caucus, colloquium,
   commission, committee, conclave, concourse, confab, confabulation,
   conference, confrontation, congregation, congress, consultation,
   conventicle, convention, convocation, council, council fire,
   council of war, dance, date, diet, discussion, eisteddfod,
   exchange of views, eyeball-to-eyeball encounter, festivity, fete,
   forgathering, forum, gathering, get-together, high-level talk,
   housewarming, huddle, interchange of views, interview, levee,
   mediumism, meet, meeting, necromancy, negotiations,
   news conference, palaver, panel, parley, party, plenum, pourparler,
   powwow, press conference, prom, quorum, rally, reception,
   rendezvous, session, shindig, sit-in, sitting, soiree, spirit,
   spiritism, spiritualism, summit, summit conference, summitry,
   symposium, synod, turnout