The Collaborative International Dictionary of English v.0.48:

Genius \Gen"ius\, n.; pl. E. Geniuses; in sense 1, L. Genii.
   [L. genius, prop., the superior or divine nature which is
   innate in everything, the spirit, the tutelar deity or genius
   of a person or place, taste, talent, genius, from genere,
   gignere, to beget, bring forth. See Gender, and cf.
   Engine.]
   1. A good or evil spirit, or demon, supposed by the ancients
      to preside over a man's destiny in life; a tutelary deity;
      a supernatural being; a spirit, good or bad. Cf. Jinnee.

   Syn: genie.
        [1913 Webster]

              The unseen genius of the wood.      --Milton.
        [1913 Webster]

              We talk of genius still, but with thought how
              changed! The genius of Augustus was a tutelary
              demon, to be sworn by and to receive offerings on
              an altar as a deity.                --Tylor.
        [1913 Webster]

   2. The peculiar structure of mind with which each individual
      is endowed by nature; that disposition or aptitude of mind
      which is peculiar to each man, and which qualifies him for
      certain kinds of action or special success in any pursuit;
      special taste, inclination, or disposition; as, a genius
      for history, for poetry, or painting.
      [1913 Webster]

   3. Peculiar character; animating spirit, as of a nation, a
      religion, a language.
      [1913 Webster]

   4. Distinguished mental superiority; uncommon intellectual
      power; especially, superior power of invention or
      origination of any kind, or of forming new combinations;
      as, a man of genius.
      [1913 Webster]

            Genius of the highest kind implies an unusual
            intensity of the modifying power.     --Coleridge.
      [1913 Webster]

   5. A man endowed with uncommon vigor of mind; a man of
      superior intellectual faculties and creativity; as,
      Shakespeare was a rare genius.

   Syn: Genius, Talent.

   Usage: Genius implies high and peculiar gifts of nature,
          impelling the mind to certain favorite kinds of mental
          effort, and producing new combinations of ideas,
          imagery, etc. Talent supposes general strength of
          intellect, with a peculiar aptitude for being molded
          and directed to specific employments and valuable ends
          and purposes. Genius is connected more or less with
          the exercise of imagination, and reaches its ends by a
          kind of intuitive power. Talent depends more on high
          mental training, and a perfect command of all the
          faculties, memory, judgment, sagacity, etc. Hence we
          speak of a genius for poetry, painting. etc., and a
          talent for business or diplomacy. Among English
          orators, Lord Chatham was distinguished for his
          genius; William Pitt for his pre["e]minent talents,
          and especially his unrivaled talent for debate.
          [1913 Webster]

   Genius loci[L.], the genius or presiding divinity of a
      place; hence, the pervading spirit of a place or
      institution, as of a college, etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

genius
    n 1: someone who has exceptional intellectual ability and
         originality; "Mozart was a child genius"; "he's smart but
         he's no Einstein" [syn: genius, mastermind, brain,
         brainiac, Einstein]
    2: unusual mental ability [syn: brilliance, genius]
    3: someone who is dazzlingly skilled in any field [syn: ace,
       adept, champion, sensation, maven, mavin,
       virtuoso, genius, hotshot, star, superstar, whiz,
       whizz, wizard, wiz]
    4: exceptional creative ability [syn: genius, wizardry]
    5: a natural talent; "he has a flair for mathematics"; "he has a
       genius for interior decorating" [syn: flair, genius]

Moby Thesaurus II by Grady Ward, 1.0:

329 Moby Thesaurus words for "genius":
   A per se, Baba Yaga, Calliope, Clio, Erato, Euterpe, Geist, Lilith,
   Melpomene, Muse, Polyhymnia, Terpsichore, Thalia, ability,
   ableness, academician, ace, acuity, acuteness, adept, adequacy,
   adroitness, afflatus, afreet, ancestral spirits, angel,
   animating spirit, animation, animus, aptitude, aptness,
   artistic imagination, astuteness, attendant godling, atua,
   barghest, bent, body-build, bookman, boss, brain, braininess,
   brains, brand, brightness, brilliance, bump, cacodemon, caliber,
   capability, capableness, capacity, cast, champion, character,
   characteristic, characteristics, chief, child prodigy, classicist,
   clear thinking, clerk, cleverness, colossus of knowledge,
   commander, competence, complexion, composition, conception,
   constituents, constitution, control, crackerjack, crasis,
   creative imagination, creative power, creative thought, creativity,
   daemon, daeva, daimonion, dean, demon, devil, dexterity, dharma,
   diathesis, disposition, divine afflatus, dower, dowry, dybbuk,
   efficacy, efficiency, endowment, enlivenment, equipment,
   esemplastic imagination, esemplastic power, esprit, ethos,
   evil genius, evil spirit, evil spirits, exhilaration, expert,
   facility, faculty, fairy godmother, familiar, familiar spirit,
   fiber, fiend, fiend from hell, fire, fire of genius, firing,
   first-rater, fitness, flair, forte, frame, fugleman, genie,
   genius domus, genius loci, ghoul, giant of learning, gift,
   gifted child, gifted person, giftedness, gifts, good angel,
   good genius, good hand, grain, grasp, great, guardian,
   guardian angel, guardian spirit, guide, gyre, habit, head, hellion,
   higher-up, household gods, hue, humanist, humor, humors, ilk,
   incubus, infection, infusion, ingenuity, inspiration, instinct,
   intellect, intellectual genius, intellectual prodigy, intelligence,
   inventiveness, invisible helper, jinni, jinniyeh, keen-wittedness,
   keenness, kind, knack, lamia, lares and penates, lares compitales,
   lares familiaris, lares permarini, lares praestites, lares viales,
   laureate, leader, leading light, learned clerk, learned man,
   literary man, litterateur, long suit, lover of learning, luminary,
   maestro, magician, mahatma, makeup, makings, man of genius,
   man of learning, man of letters, man of parts, manes, master,
   master hand, mastermind, mental alertness, mental genius,
   mental giant, mercurial mind, metier, mine of information,
   ministering angel, mold, moving spirit, muse, mythicization,
   mythification, mythopoeia, native cleverness, natural,
   natural endowment, natural gift, nature, nimble mind,
   nimble-wittedness, nimbleness, nonpareil, nous, numen, originality,
   paragon, parts, past master, penates, philologist, philologue,
   philomath, philosophe, philosopher, physique, poetic imagination,
   polyhistor, polymath, potential, power, powers, practiced hand,
   principal, prodigy, proficiency, property, pundit, qualification,
   quality, quick parts, quick thinking, quick wit, quick-wittedness,
   quickness, rakshasa, ready wit, ruler, sage, satan, savant, savvy,
   scholar, scholastic, schoolman, senior, shaping imagination,
   sharp-wittedness, sharpness, shedu, skilled hand, smartness,
   smarts, somatotype, sort, soul, special providence, speciality,
   specter, spirit, sprightly wit, stamp, star, streak, stripe,
   strong flair, strong point, student, succubus, suchness,
   sufficiency, superior, superman, supernatural being, superstar,
   susceptibility, system, talent, talents, temper, temperament,
   tendency, tenor, the goods, the greatest, the most, the stuff,
   the undead, tone, top dog, topnotcher, totem, turn, tutelar god,
   tutelary, type, understanding, vampire, vein, virtuoso,
   walking encyclopedia, way, what it takes, whiz, wit, wizard,
   wonder, yogini