The Collaborative International Dictionary of English v.0.48:

Aptitude \Apt"i*tude\, n. [F. aptitude, LL. aptitudo, fr. L.
   aptus. See Apt, and cf. Attitude.]
   1. A natural or acquired disposition or capacity for a
      particular purpose, or tendency to a particular action or
      effect; as, oil has an aptitude to burn.
      [1913 Webster]

            He seems to have had a peculiar aptitude for the
            management of irregular troops.       --Macaulay.
      [1913 Webster]

   2. A general fitness or suitableness; adaptation.
      [1913 Webster]

            That sociable and helpful aptitude which God
            implanted between man and woman.      --Milton.
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   3. Readiness in learning; docility; aptness.
      [1913 Webster]

            He was a boy of remarkable aptitude.  --Macaulay.
      [1913 Webster]

WordNet (r) 3.0 (2006):

aptitude
    n 1: inherent ability [ant: inaptitude]

Moby Thesaurus II by Grady Ward, 1.0:

160 Moby Thesaurus words for "aptitude":
   a thing for, ability, acuity, acuteness, admissibility, adroitness,
   affinity, an ear for, an eye for, animus, applicability,
   appositeness, appropriateness, aptness, bent, bias, braininess,
   brightness, brilliance, capability, capacity, capacity for, cast,
   chance, character, clear thinking, cleverness, conatus,
   conduciveness, constitution, contingency, delight, dexterity,
   diathesis, disposition, docility, eagerness, eccentricity,
   educability, esprit, eventuality, expectation, facility, faculty,
   fair expectation, favorable prospect, feeling for, felicity,
   fitness, fittedness, flair, genius, genius for, gift, gift for,
   giftedness, gifts, good chance, grain, idiosyncrasy,
   impressionability, inclination, individualism, innate aptitude,
   intellect, intelligence, keen-wittedness, keenness, kidney,
   leaning, liability, liableness, likelihood, likeliness, liking,
   make, makeup, malleability, mental alertness, mental set,
   mercurial mind, mettle, mind, mind-set, mold, moldability,
   motivation, native cleverness, nature, nimble mind,
   nimble-wittedness, nimbleness, nous, obligation, odds, outlook,
   penchant, plasticity, pliability, possibility, predilection,
   predisposition, preference, prejudice, presumption,
   presumptive evidence, probabilism, probability, proclivity,
   proneness, propensity, propriety, prospect, qualification,
   quick parts, quick thinking, quick wit, quick-wittedness,
   quickness, readiness, ready wit, reasonable ground,
   reasonable hope, receptivity, relevance, savvy, sensitivity to,
   set, sharp-wittedness, sharpness, slant, smartness, smarts,
   soft spot, sprightly wit, stamp, strain, streak, stripe,
   suitability, suitableness, susceptibility, talent, teachability,
   teachableness, temper, temperament, tendency, trainableness,
   tropism, turn, turn for, turn of mind, twist, type, verisimilitude,
   warp, weakness, well-grounded hope, willingness