The Collaborative International Dictionary of English v.0.48:

Ability \A*bil"i*ty\ ([.a]*b[i^]l"[i^]*t[y^]), n.; pl.
   Abilities ([.a]*b[i^]l"[i^]*t[i^]z). [F. habilet['e],
   earlier spelling habilit['e] (with silent h), L. habilitas
   aptitude, ability, fr. habilis apt. See Able.]
   The quality or state of being able; power to perform, whether
   physical, moral, intellectual, conventional, or legal;
   capacity; skill or competence in doing; sufficiency of
   strength, skill, resources, etc.; -- in the plural, faculty,
   talent.
   [1913 Webster]

         Then the disciples, every man according to his ability,
         determined to send relief unto the brethren. --Acts xi.
                                                  29.
   [1913 Webster]

         Natural abilities are like natural plants, that need
         pruning by study.                        --Bacon.
   [1913 Webster]

         The public men of England, with much of a peculiar kind
         of ability.                              --Macaulay.
   [1913 Webster]

   Syn: Capacity; talent; cleverness; faculty; capability;
        efficiency; aptitude; aptness; address; dexterity;
        skill.

   Usage: Ability, Capacity. These words come into
          comparison when applied to the higher intellectual
          powers. Ability has reference to the active exercise
          of our faculties. It implies not only native vigor of
          mind, but that ease and promptitude of execution which
          arise from mental training. Thus, we speak of the
          ability with which a book is written, an argument
          maintained, a negotiation carried on, etc. It always
          something to be done, and the power of doing it.
          Capacity has reference to the receptive powers. In its
          higher exercises it supposes great quickness of
          apprehension and breadth of intellect, with an
          uncommon aptitude for acquiring and retaining
          knowledge. Hence it carries with it the idea of
          resources and undeveloped power. Thus we speak of the
          extraordinary capacity of such men as Lord Bacon,
          Blaise Pascal, and Edmund Burke. "Capacity," says H.
          Taylor, "is requisite to devise, and ability to
          execute, a great enterprise." The word abilities, in
          the plural, embraces both these qualities, and denotes
          high mental endowments.
          [1913 Webster] Abime

WordNet (r) 3.0 (2006):

ability
    n 1: the quality of being able to perform; a quality that
         permits or facilitates achievement or accomplishment [ant:
         inability, unfitness]
    2: possession of the qualities (especially mental qualities)
       required to do something or get something done; "danger
       heightened his powers of discrimination" [syn: ability,
       power] [ant: inability]

Moby Thesaurus II by Grady Ward, 1.0:

132 Moby Thesaurus words for "ability":
   ableness, address, adeptness, adequacy, adroitness, airmanship,
   aptitude, aptness, artfulness, artisanship, artistry, bravura,
   brilliance, bump, caliber, capability, capableness, capacity,
   capital, cleverness, command, competence, competency, condition,
   control, coordination, craft, craftsmanship, cunning, deftness,
   devices, dexterity, dexterousness, dextrousness, diplomacy,
   disposable resources, dower, dowry, efficacy, efficiency,
   endowment, equipment, expertise, expertism, expertness, facility,
   faculty, finesse, fitness, fittedness, flair, forte, funds, genius,
   gift, grace, grip, handiness, horsemanship, ingeniousness,
   ingenuity, instinct, knack, know-how, long suit, makings,
   marksmanship, mastership, mastery, maturity, means, method, metier,
   might, natural endowment, natural gift, parts, potential, power,
   powers, practical ability, preparedness, proficiency, prowess,
   qualification, quickness, readiness, recourses, resorts, resource,
   resourcefulness, resources, ripeness, savoir-faire, savvy,
   seamanship, seasoning, skill, skillfulness, speciality, stock,
   strong flair, strong point, style, sufficiency, suitability,
   suitableness, suitedness, supply, susceptibility, tact,
   tactfulness, talent, talents, technical brilliance,
   technical mastery, technical skill, technique, tempering,
   the goods, the stuff, timing, trim, virtuosity, ways,
   ways and means, what it takes, wherewith, wherewithal, wit,
   wizardry, workmanship



The Devil's Dictionary (1881-1906):

ABILITY, n.  The natural equipment to accomplish some small part of
the meaner ambitions distinguishing able men from dead ones.  In the
last analysis ability is commonly found to consist mainly in a high
degree of solemnity.  Perhaps, however, this impressive quality is
rightly appraised; it is no easy task to be solemn.