The Collaborative International Dictionary of English v.0.48:

Faction \Fac"tion\ (f[a^]k"sh[u^]n), n. [L. factio a doing, a
   company of persons acting together, a faction: cf. F. faction
   See Fashion.]
   1. (Anc. Hist.) One of the divisions or parties of
      charioteers (distinguished by their colors) in the games
      of the circus.
      [1913 Webster]

   2. A party, in political society, combined or acting in
      union, in opposition to the government, or state; --
      usually applied to a minority, but it may be applied to a
      majority; a combination or clique of partisans of any
      kind, acting for their own interests, especially if
      greedy, clamorous, and reckless of the common good.
      [1913 Webster]

   3. Tumult; discord; dissension.
      [1913 Webster]

            They remained at Newbury in great faction among
            themselves.                           --Clarendon.

   Syn: Combination; clique; junto. See Cabal.
        [1913 Webster]

WordNet (r) 3.0 (2006):

faction
    n 1: a clique (often secret) that seeks power usually through
         intrigue [syn: cabal, faction, junto, camarilla]
    2: a dissenting clique [syn: faction, sect]

Moby Thesaurus II by Grady Ward, 1.0:

207 Moby Thesaurus words for "faction":
   American Party, Anti-Monopoly Party, Bull Moose Party,
   Communist Party, Conservative Party, Constitutional Union Party,
   Democratic Party, Democratic-Republican Party, Farmer-Labor Party,
   Federalist Party, Free Soil Party, Greenback Party,
   Know-Nothing Party, Labour Party, Liberal Party,
   Liberal Republican Party, Liberty Party, National Republican Party,
   Populist Party, Progressive Party, Prohibition Party,
   Republican Party, Socialist Labor Party, Socialist Party,
   Socialist Workers Party, Tammany Hall, Tory Party, Whig Party,
   affiliation, age group, aggressiveness, antagonism, argumentation,
   band, battalion, bellicosity, belligerence, bevy, bickering, bloc,
   body, branch, breakaway group, brigade, bunch, cabal, cadre,
   camarilla, camp, cast, caucus, church, circle, city hall,
   clannishness, clanship, clash, clashing, clique, cliqueyness,
   cliquishness, cliquism, coalition, cohort, combativeness, combine,
   communion, community, company, complement, conflict, contention,
   contentiousness, contingent, contradiction, controversy, corps,
   coterie, covey, crew, crowd, denomination, denominationalism,
   detachment, detail, difference, disaccord, disaccordance,
   disagreement, discord, discordance, discordancy, discrepancy,
   disharmony, disparity, dissension, dissent, dissidence, dissonance,
   disunion, disunity, divergence, diversity, division,
   esprit de corps, ethnic group, ethnocentricity, exclusiveness,
   factionalism, factiousness, fellowship, flak, fleet, gang, group,
   grouping, groupment, in-group, inaccordance, inequality,
   infighting, interest, interest group, intrigue, irascibility,
   irritability, jarring, junta, junto, litigiousness, lobby, machine,
   major party, minor party, minority group, mob, movement,
   multiple party system, negation, offshoot, one-party system,
   opposition, opposition party, oppugnancy, order, organization,
   out-group, outfit, pack, part, partiality, partisan spirit,
   partisanism, partisanship, party, party in power, party spirit,
   peer group, persuasion, phalanx, platoon, political machine,
   political party, posse, pressure group, pugnacity, quarrelsomeness,
   regiment, religious order, repugnance, rift, ring, rupture, salon,
   schism, school, sect, sectarianism, sectarism, sectionalism,
   sedition, segment, set, shrewishness, side, silent majority,
   society, splinter, splinter group, splinter party, split, squad,
   stable, strife, string, team, third party, tribe, troop, troupe,
   two-party system, unharmoniousness, variance, variety, version,
   vocal minority, wing