The Collaborative International Dictionary of English v.0.48:

Champion \Cham"pi*on\ (ch[a^]m"p[i^]*[u^]n), n. [F. champion,
   fr. LL.campio, of German origin; cf. OHG. chempho, chemphio,
   fighter, champf, G. kampf, contest; perh. influenced by L.
   campus field, taken in the sense of "field of battle."]
   1. One who engages in any contest; especially one who in
      ancient times contended in single combat in behalf of
      another's honor or rights; or one who now acts or speaks
      in behalf of a person or a cause; a defender; an advocate;
      a hero.
      [1913 Webster]

            A stouter champion never handled sword. --Shak.
      [1913 Webster]

            Champions of law and liberty.         --Fisher Ames.
      [1913 Webster]

   2. One who by defeating all rivals, has obtained an
      acknowledged supremacy in any branch of athletics or game
      of skill, and is ready to contend with any rival; as, the
      champion of England.
      [1913 Webster]

   Note: Champion is used attributively in the sense of
         surpassing all competitors; overmastering; as, champion
         pugilist; champion chess player.

   Syn: Leader; chieftain; combatant; hero; warrior; defender;
        protector.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Champion \Cham"pi*on\, v. t. [imp. & p. p. Championed; p. pr.
   & vb. n. Championing.] [Obs.] --Shak.
   [1913 Webster]
   2. To furnish with a champion; to attend or defend as
      champion; to support or maintain; to protect.
      [1913 Webster]

            Championed or unchampioned, thou diest. --Sir W.
                                                  Scott.
      [1913 Webster]

WordNet (r) 3.0 (2006):

champion
    adj 1: holding first place in a contest; "a champion show dog";
           "a prizewinning wine" [syn: champion, prizewinning]
    n 1: someone who has won first place in a competition [syn:
         champion, champ, title-holder]
    2: someone who fights for a cause [syn: champion, fighter,
       hero, paladin]
    3: a person who backs a politician or a team etc.; "all their
       supporters came out for the game"; "they are friends of the
       library" [syn: supporter, protagonist, champion,
       admirer, booster, friend]
    4: someone who is dazzlingly skilled in any field [syn: ace,
       adept, champion, sensation, maven, mavin,
       virtuoso, genius, hotshot, star, superstar, whiz,
       whizz, wizard, wiz]
    v 1: protect or fight for as a champion [syn: champion,
         defend]

Moby Thesaurus II by Grady Ward, 1.0:

294 Moby Thesaurus words for "champion":
   A per se, Maecenas, Olympic medal winner, abettor, ace, admirer,
   advocate, aficionado, allege in support, alter ego, alternate,
   amicus curiae, angel, answer, apologete, apologist, apologizer,
   arch, argue for, arm, armor, assert, attorney, award winner, back,
   back up, backer, backstop, backup, backup man, bang-up, banner,
   battle, best, bless, blue-ribbon, boss, buff, campaigner, capital,
   cardinal, central, champ, chief, choice, chosen, cloak, combatant,
   commander, compass about, conquering hero, conqueror, conquistador,
   contend, contend for, copyright, counter, cover, cream, crowning,
   cushion, dandy, dean, defeater, defend, defender, dependence,
   deputy, distinguished, dominant, dummy, easy winner, elect, elite,
   encourager, endorser, ensure, espouse, executive officer, exponent,
   expounder, fan, fat, favorer, fence, fend, fight for, fighter,
   figurehead, first, flank, flower, focal, for the best, foremost,
   forward, friend at court, fugleman, genius, get behind,
   get in behind, great, greatest, guarantee, guard, guard against,
   guardian, handpicked, harbor, haven, head, headmost, hegemonic,
   hero, higher-up, illustrious, insure, justifier, keep,
   keep from harm, laureate, leader, leading, lieutenant, locum,
   locum tenens, lover, magisterial, main, mainstay, maintain,
   maintainer, make a plea, make safe, master, matchless, medalist,
   nestle, nonesuch, nonpareil, optimal, optimum, outstanding,
   overruling, paladin, pancratiast, paragon, paramount, paranymph,
   partisan, patent, patron, patroness, peerless, pick, picked,
   pillar, pinch hitter, plead for, pleader, police, predominant,
   preeminent, premier, preponderant, prevailing, primal, primary,
   prime, principal, prize, prizeman, prizetaker, prizewinner,
   procurator, prodigy, promote, promoter, proponent, protagonist,
   protect, protector, protectress, proxy, queen, quintessence,
   quintessential, ranking, rebut, refute, register, reliance, reply,
   representative, respond, ride shotgun for, riposte, rock, ruler,
   ruling, run interference for, runner-up, safeguard, safekeeper,
   say in defense, screen, second, second in command, secondary,
   seconder, sectary, secure, select, senior, shelter, shield,
   shoo-in, shroud, side with, sider, sovereign, speak for,
   speak up for, splendid, sponsor, stalwart, stand back of,
   stand behind, stand by, stand up for, stand-in, standby, star,
   stellar, stick by, stick up for, subduer, subjugator, substitute,
   successful advocate, supereminent, superior, superlative, superman,
   superstar, support, supporter, supreme, sure winner, surpassing,
   surrogate, sustain, sustainer, sympathizer, take sides with,
   the best, the best ever, the greatest, the most, the tops,
   the very best, top dog, top-notch, topflight, tower,
   tower of strength, triumpher, understudy, underwrite, unmatchable,
   unmatched, unparalleled, unsurpassed, uphold, upholder,
   urge reasons for, utility man, vanquisher, very best, veteran,
   vicar, vicar general, vice, vicegerent, victor, vindicator,
   virtuoso, votary, warrior, well-wisher, whitewasher, winner,
   world champion, world-record holder



Easton's 1897 Bible Dictionary:

Champion
   (1 Sam. 17:4, 23), properly "the man between the two," denoting
   the position of Goliath between the two camps. Single combats of
   this kind at the head of armies were common in ancient times. In
   ver. 51 this word is the rendering of a different Hebrew word,
   and properly denotes "a mighty man."


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

CHAMPION. He who fights for another, or takes his place in a quarrel; it
also includes him who fights his own battles. Bract. lib. 4, t. 2, c. 12.