The Collaborative International Dictionary of English v.0.48:

Adventurer \Ad*ven"tur*er\, n. [Cf. F. aventurier.]
   [1913 Webster]
   1. One who adventures; as, the merchant adventurers; one who
      seeks his fortune in new and hazardous or perilous
      enterprises.
      [1913 Webster]

   2. A social pretender on the lookout for advancement.
      [1913 Webster]

WordNet (r) 3.0 (2006):

adventurer
    n 1: a person who enjoys taking risks [syn: adventurer,
         venturer]
    2: someone who travels into little known regions (especially for
       some scientific purpose) [syn: explorer, adventurer]

Moby Thesaurus II by Grady Ward, 1.0:

131 Moby Thesaurus words for "adventurer":
   adventuress, alpinist, astronaut, betting ring, bettor,
   big operator, boneshaker, bounder, bourgeois gentilhomme,
   brazenface, cad, camper, cardshark, cardsharp, cardsharper,
   charlatan, cheat, climber, comers and goers, commuter,
   compulsive gambler, condottiere, cosmopolite, crap shooter,
   cruiser, daredevil, devil, excursionist, explorer, fare,
   fire-eater, free lance, gambler, gamester, globe-girdler,
   globe-trotter, goer, gun, gunman, gunslinger, hajji, harum-scarum,
   hazarder, hellcat, hero, heroine, hired gun, hired killer,
   hireling, hotspur, jet set, jet-setter, journeyer, knave,
   lame duck, madbrain, madcap, margin purchaser, mariner, mercenary,
   mountaineer, name-dropper, newly-rich, nouveau riche,
   nouveau roturier, operator, opportunist, palmer, parvenu,
   passenger, passerby, pathfinder, petty gambler, philanderer,
   pig in clover, piker, pilgrim, pioneer, player, plunger,
   professional killer, punter, rantipole, rogue, rubberneck,
   rubbernecker, sailor, scalper, scoundrel, sharp, sharper, sharpie,
   sightseer, smart operator, social climber, soldier of fortune,
   speculator, sport, sporting man, sportsman, sprout, stag,
   status seeker, straphanger, swashbuckler, swindler, tinhorn,
   tipster, tourer, tourist, tout, trailblazer, trailbreaker,
   transient, traveler, trekker, trickster, tripper, tufthunter,
   upstart, venturer, viator, visiting fireman, voortrekker, voyager,
   voyageur, wagerer, wayfarer, wild man, world-traveler,
   would-be gentleman