The Collaborative International Dictionary of English v.0.48:

Posse \Pos"se\, n.
   See Posse comitatus.
   [1913 Webster]

   In posse. See In posse in the Vocabulary.
      [1913 Webster]

WordNet (r) 3.0 (2006):

posse
    n 1: a temporary police force [syn: posse, posse comitatus]

Moby Thesaurus II by Grady Ward, 1.0:

128 Moby Thesaurus words for "posse":
   Cheka, FBI, Gestapo, Interpol, KP, MP, MVD, Mounties, NKVD, OGPU,
   RCMP, SP, Scotland Yard, age group, army, army group, band,
   battalion, battery, battle group, bevy, body, brigade, bunch,
   cabal, cadre, cast, clique, cohort, column, combat command,
   combat team, company, complement, constabulary, contingent, corps,
   coterie, county police, covey, crew, crowd, detachment, detail,
   division, domiciliary visit, dragnet, exploration, faction,
   field army, field train, file, fleet, flying column, forage, frisk,
   gang, garrison, group, grouping, groupment, highway patrol,
   house-search, hunt, hunting, in-group, junta, kitchen police,
   law enforcement agency, legion, maniple, military police, mob,
   movement, organization, out-group, outfit, pack, party, peer group,
   perquisition, phalanx, platoon, police, police force,
   political police, probe, provincial police, quest, rank,
   ransacking, regiment, riot police, rummage, salon, search,
   search party, search warrant, search-and-destroy operation,
   searching, secret police, section, security force, set,
   shore patrol, special police, squad, squadron, stable, stalk,
   stalking, state police, still hunt, string, tactical police,
   tactical unit, task force, team, train, tribe, troop, troopers,
   troupe, turning over, unit, vigilance committee, vigilantes,
   wing



Bouvier's Law Dictionary, Revised 6th Ed (1856):

POSSE. This word is used substantively to signify a possibility. For
example, such a thing is in posse, that is, such a thing may possibly be;
when the thing is in being, the phrase to express it is, in esse. (q.v.)