The Collaborative International Dictionary of English v.0.48:

Joker \Jok"er\, n.
   1. One who makes jokes or jests.
      [1913 Webster]

   2. (Card Playing) See Best bower, under 2d Bower.
      [1913 Webster]

   3. (Card Playing) An extra card usually included in a deck of
      playing cards, having the same design as the others on the
      back, but on the face having a picture of a jester. It is
      not included in the deck used in most games, but in
      certain games may be included and then takes on a special
      value, such as the highest-valued card, or a wild card.
      [PJC]

   4. A clause placed in a document, such as a contract or a
      piece of legislation, not itself appearing significant,
      but in a subtle way substantially changing the effect of
      the document.
      [PJC]

   5. Hence: Any fact or condition which is unknown or not
      apparent, which reverses an apparently advantageous
      position; a kicker.
      [PJC]

   6. A person; a fellow; a chap; -- usually used in a mildly
      disparaging sense; as, who's the joker who left the ice
      cream on the table?.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Bower \Bow"er\ (bou"[~e]r), n. [G. bauer a peasant. So called
   from the figure sometimes used for the knave in cards. See
   Boor.]
   One of the two highest cards in the pack commonly used in the
   game of euchre.
   [1913 Webster]

   Right bower, the knave of the trump suit, the highest card
      (except the "Joker") in the game.

   Left bower, the knave of the other suit of the same color
      as the trump, being the next to the right bower in value.


   Best bower or Joker, in some forms of euchre and some
      other games, an extra card sometimes added to the pack,
      which takes precedence of all others as the highest card.
      [1913 Webster]

WordNet (r) 3.0 (2006):

joker
    n 1: a person who enjoys telling or playing jokes [syn: joker,
         jokester]
    2: a person who does something thoughtless or annoying; "some
       joker is blocking the driveway" [syn: joker, turkey]
    3: an inconspicuous clause in a document or bill that affects
       its meaning in a way that is not immediately apparent; "when
       I demanded my money he showed me the joker in the contract"
    4: a playing card that is usually printed with a picture of a
       jester

Moby Thesaurus II by Grady Ward, 1.0:

305 Moby Thesaurus words for "joker":
   Adamite, Artful Dodger, Casanova, Don Juan, Machiavel, Machiavelli,
   Machiavellian, ace, actor, artful dodge, artifice, astonishment,
   bad boy, bag of tricks, bamboozler, banana, bastard, befuddler,
   beguiler, being, best bower, bill, bird, blind, block, blockade,
   blockbuster, blow, bluff, body, bomb, bombshell, booger, bosey,
   bottleneck, boundary condition, bower, buffoon, bugger, burlesquer,
   calendar, card, cards, caricaturist, cat, catch, catch-22, chap,
   character, charmer, chicanery, chouse, clause, clown, clubs,
   comedian, comedienne, comic, companion bills amendment, condition,
   cordon, counterfeiter, creature, crux, curtain, curve, curve-ball,
   customer, cutup, deceiver, deck, deluder, design, determent,
   deterrent, deuce, device, devil, diamonds, difficulty, dirty deal,
   dirty trick, dissembler, dissimulator, dodge, dodger, donnee,
   double-dealer, dragnet clause, drawback, droll, duck, dummy, duper,
   earthling, earthshaker, elf, enacting clause, enchanter,
   enfant terrible, entrancer, epigrammatist, escalator clause,
   escape clause, escape hatch, eye-opener, face cards, faker,
   farceur, fast deal, feint, feller, fellow, fetch, ficelle,
   fine print, flush, fooler, forger, full house, funmaker, funnyman,
   gag writer, gagman, gagster, gambit, gay deceiver, gimmick, given,
   googly, groundling, grounds, guy, hand, hang-up, hazard, head,
   hearts, hitch, hoaxer, hocus-pocus, hold-up bill, homo, hood,
   hoodlum, hooligan, human, human being, humorist, hurdle,
   hypnotizer, imp, individual, ironist, jack, jasper, jester, jilt,
   jilter, jokesmith, jokester, juggle, kicker, kidder, king, knave,
   lad, lampooner, left bower, leg-puller, life, limiting condition,
   little devil, little monkey, little rascal, living soul, madcap,
   man, mesmerizer, minx, mischief, mischief-maker, misleader, mortal,
   motion, nose, objection, obligation, obstacle, obstruction,
   obstructive, omnibus bill, one, one small difficulty, pack, pair,
   parameter, parodist, party, pass, peripeteia, person, personage,
   personality, picture cards, pinch, pitfall, pixie, plagiarist,
   plagiarizer, playactor, playing cards, ploy, practical joker,
   prankster, prerequisite, privileged question, provision,
   provisions, proviso, puck, punner, punster, queen, question,
   quipster, ragger, rapscallion, rascal, reparteeist, requisite,
   revelation, rider, rogue, role-player, round, rowdy, royal flush,
   rub, rubber, ruff, ruffian, ruse, satirist, saving clause, scamp,
   scapegrace, scheme, scurvy trick, seducer, shift, shocker,
   sine qua non, single, singleton, sleight, sleight of hand,
   sleight-of-hand trick, small print, snag, somebody, someone, soul,
   spades, specification, spoofer, staggerer, startler, stipulation,
   straight, stratagem, string, stud, stumbling block,
   stumbling stone, subterfuge, surprisal, surprise, surprise ending,
   surprise package, surprise party, switch, tease, teaser, tellurian,
   terms, terran, thunderbolt, thunderclap, trap, trey, trick,
   trickster, trump, twist, ultimatum, wag, wagwit, whereas, wile,
   wisecracker, wit, witling, worldling, zany