The Collaborative International Dictionary of English v.0.48:

Lion \Li"on\ (l[imac]"[u^]n), n. [F. lion, L. leo, -onis, akin
   to Gr. le`wn. Cf. Chameleon, Dandelion, Leopard.]
   1. (Zool.) A large carnivorous feline mammal (Panthera leo,
      formerly Felis leo), found in Southern Asia and in most
      parts of Africa, distinct varieties occurring in the
      different countries. The adult male, in most varieties,
      has a thick mane of long shaggy hair that adds to his
      apparent size, which is less than that of the largest
      tigers. The length, however, is sometimes eleven feet to
      the base of the tail. The color is a tawny yellow or
      yellowish brown; the mane is darker, and the terminal tuft
      of the tail is black. In one variety, called the maneless
      lion, the male has only a slight mane.
      [1913 Webster]

   2. (Astron.) A sign and a constellation; Leo.
      [1913 Webster]

   3. An object of interest and curiosity, especially a person
      who is so regarded; as, he was quite a lion in London at
      that time.
      [1913 Webster]

            Such society was far more enjoyable than that of
            Edinburgh, for here he was not a lion, but a man.
                                                  --Prof.
                                                  Wilson.
      [1913 Webster]

   American lion (Zool.), the puma or cougar.

   Lion ant (Zool.), the ant-lion.

   Lion dog (Zool.), a fancy dog with a flowing mane, usually
      clipped to resemble a lion's mane.

   Lion lizard (Zool.), the basilisk.

   Lion's share, all, or nearly all; the best or largest part;
      -- from Aesop's fable of the lion hunting in company with
      certain smaller beasts, and appropriating to himself all
      the prey.

   Lion of Lucerne, a famous sculptured lion at Lucerne,
      Switzerland, designed by Thorwaldsen and dedicated in 1821
      as a memorial to the Swiss Guards who fell defending Louis
      XVI. in the attack of the mob on the Tuileries, Aug. 10,
      1792. The animal, which is hewn out of the face of a rock,
      is represented as transfixed with a broken spear and
      dying, but still trying to protect with its paw a shield
      bearing the fleur-de-lis of France.

   Lion of St. Mark, a winged lion, the emblem of the
      evangelist Mark, especially that of bronze surmounting a
      granite column in the Piazzetta at Venice, and holding in
      its fore paws an open book representing St. Mark's Gospel.


   Lion of the North, Gustavus Adolphus (1594-1632), King of
      Sweden, the hero of the Protestant faith in the Thirty
      Years' War.
      [1913 Webster + Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

lion
    n 1: large gregarious predatory feline of Africa and India
         having a tawny coat with a shaggy mane in the male [syn:
         lion, king of beasts, Panthera leo]
    2: a celebrity who is lionized (much sought after) [syn: lion,
       social lion]
    3: (astrology) a person who is born while the sun is in Leo
       [syn: Leo, Lion]
    4: the fifth sign of the zodiac; the sun is in this sign from
       about July 23 to August 22 [syn: Leo, Leo the Lion,
       Lion]

Moby Thesaurus II by Grady Ward, 1.0:

220 Moby Thesaurus words for "lion":
   Achilles, David, Establishment, Gibraltar, Hector, Leo, Roland,
   Samson, Siberian tiger, VIP, a man, achievement, alerion,
   animal charge, annulet, argent, armorial bearings, armory, arms,
   azure, bandeau, bar, bar sinister, baron, baton, bearings, bend,
   bend sinister, big boy, big gun, big man, big name, big-timer,
   bigwig, billet, blazon, blazonry, bobcat, bordure, brass,
   brass hat, brave, broad arrow, bulldog, cadency mark, canton,
   cat-a-mountain, catamount, celebrity, chaplet, charge, cheetah,
   chevron, chief, chutzpanik, coat of arms, cockatrice,
   constellation, coronet, cougar, crescent, crest, cross,
   cross moline, crown, cynosure, decorated hero, demigod,
   demigoddess, device, difference, differencing, dignitary, dignity,
   eagle, elder, eminence, ermine, ermines, erminites, erminois,
   escutcheon, falcon, father, fess, fess point, field, fighting cock,
   figure, file, flanch, fleur-de-lis, folk hero, fret, fur, fusil,
   galaxy, gallant, gamecock, garland, good soldier, great man,
   griffin, gules, gyron, hatchment, heart of oak, helmet,
   heraldic device, hero, heroine, high-muck-a-muck, honor point,
   horse, idol, immortal, impalement, impaling, important person,
   inescutcheon, interests, iron, jaguar, label, leader, leopard,
   lords of creation, lozenge, luminaries, luminary, lynx, magnate,
   man of courage, man of mark, mantling, marshaling, martlet, mascle,
   master spirit, metal, mogul, motto, mountain lion, mullet, nabob,
   nails, name, nombril point, notability, notable, oak, ocelot,
   octofoil, or, ordinary, orle, ox, painter, paladin, pale, paly,
   panjandrum, panther, pean, person of note, person of renown,
   personage, personality, pheon, pillar of society, pleiad, pop hero,
   popular hero, popular idol, power, power elite, public figure,
   puma, purpure, quarter, quartering, rock, rose, ruling circle,
   sable, sachem, saltire, scutcheon, shield, simba, social lion,
   somebody, something, spread eagle, stalwart, star, steel,
   subordinary, superstar, tenne, the brave, the great, the top,
   tiger, tincture, top brass, top people, torse, tressure, tycoon,
   unicorn, vair, valiant, valiant knight, vert,
   very important person, wildcat, worthy, wreath, yale