The Collaborative International Dictionary of English v.0.48:

Leader \Lead"er\, n.
   1. One who, or that which, leads or conducts; a guide; a
      conductor. Especially:
      (a) One who goes first.
      (b) One having authority to direct; a chief; a commander.
      (c) (Mus.) A performer who leads a band or choir in music;
          also, in an orchestra, the principal violinist; the
          one who plays at the head of the first violins.
      (d) (Naut.) A block of hard wood pierced with suitable
          holes for leading ropes in their proper places.
      (e) (Mach.) The principal wheel in any kind of machinery.
          [Obs. or R.] --G. Francis.
      (f) A horse placed in advance of others; one of the
          forward pair of horses.
          [1913 Webster]

                He forgot to pull in his leaders, and they
                gallop away with him at times.    --Hare.
      (g) A pipe for conducting rain water from a roof to a
          cistern or to the ground; a conductor.
      (h) (Fishing) A net for leading fish into a pound, weir,
          etc.; also, a line of gut, to which the snell of a fly
          hook is attached.
      (i) (Mining) A branch or small vein, not important in
          itself, but indicating the proximity of a better one.
          [1913 Webster]

   2. The first, or the principal, editorial article in a
      newspaper; a leading or main editorial article.
      [1913 Webster]

   3. (Print.)
      (a) A type having a dot or short row of dots upon its
          face.
      (b) pl. a row of dots, periods, or hyphens, used in tables
          of contents, etc., to lead the eye across a space to
          the right word or number.
          [1913 Webster]

   Syn: chief; chieftain; commander. See Chief.
        [1913 Webster]

WordNet (r) 3.0 (2006):

leader
    n 1: a person who rules or guides or inspires others [ant:
         follower]
    2: a featured article of merchandise sold at a loss in order to
       draw customers [syn: drawing card, loss leader, leader]

Moby Thesaurus II by Grady Ward, 1.0:

247 Moby Thesaurus words for "leader":
   A per se, AG, DA, KC, Kapellmeister, Mahdi, QC, SSC, US attorney,
   ace, analysis, ancestor, announcer, antecedent, article,
   article of commerce, article of merchandise, attorney general,
   avant-garde, band leader, band major, bandmaster, bell cow,
   bell mare, bellwether, bidet, big cheese, big wheel, big-timer,
   bigwig, bone of contention, born leader, boss, buccinator,
   bushwhacker, captain, carriage horse, cart horse, catechism,
   cavalry horse, chairman, champion, charismatic leader, chief,
   chieftain, choirmaster, choragus, civilian, cock, commandant,
   commander, comment, commentary, commentation, commodity, conductor,
   corporation lawyer, coryphaeus, court-appointed lawyer,
   criminal lawyer, critical review, criticism, critique,
   cross-interrogatory, cross-question, dean, debating point,
   defense counsel, demand, dignitary, director, district attorney,
   doyen, doyenne, draft horse, dray horse, driving horse, drug,
   drum major, duce, editorial, eminence, explorer, feature, feeler,
   file leader, fill horse, filler, forebear, foregoer, foreman,
   forerunner, front runner, frontiersman, fugleman, gaffer, general,
   genius, gigster, gloss, groundbreaker, guide, hack, hackney,
   harbinger, head, headman, herald, hierarch, high priest, higher-up,
   honcho, hunter, important person, innovator, inquiry,
   inspired leader, interrogation, interrogative, interrogatory,
   issue, item, jument, junior counsel, kingfish, kingpin, laureate,
   law agent, lead, lead item, lead runner, leader of men,
   leading article, leading card, leading question, lion, loss leader,
   luminary, main feature, manager, master, messenger, messiah,
   moot point, mount, mouthpiece, nonpareil, notability, notice,
   number one, orchestra leader, pacemaker, pacesetter, pack horse,
   palfrey, paragon, pathfinder, personage, pilot, pioneer,
   plow horse, point, point at issue, point in question, pole horse,
   polo pony, post-horse, precedent, precentor, precursor,
   predecessor, principal, private attorney, problem, prodigy,
   product, prosecuting attorney, prosecutor, public prosecutor,
   publicist, query, question, question at issue, question mark,
   quodlibet, rector, remark, remount, report, review, rider,
   riding horse, ringleader, road horse, roadster, rouncy, ruler,
   running commentary, saddle horse, saddler, scout, seconds, senior,
   shaft horse, silk, silk gown, solicitor general, special,
   special pleader, stalking-horse, standard article, standard-bearer,
   staple, staple item, star, stormy petrel, straw boss, stuff gown,
   stuff-gownsman, sumpter, sumpter horse, superintendent, superior,
   superman, superstar, symphonic conductor, the greatest, the most,
   thill horse, thiller, top dog, topic, torchbearer, trailblazer,
   trailbreaker, trial balloon, vanguard, vaunt-courier, vendible,
   vexed question, virtuoso, voortrekker, ware, wheeler, wheelhorse,
   workhorse, write-up