The Collaborative International Dictionary of English v.0.48:

Fitness \Fit"ness\, n.
   The state or quality of being fit; as, the fitness of
   measures or laws; a person's fitness for office.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fitness
    n 1: the quality of being suitable; "they had to prove their
         fitness for the position" [syn: fitness, fittingness]
         [ant: unfitness]
    2: good physical condition; being in shape or in condition [syn:
       fitness, physical fitness] [ant: softness, unfitness]
    3: fitness to traverse the seas [syn: seaworthiness,
       fitness]
    4: the quality of being qualified

Moby Thesaurus II by Grady Ward, 1.0:

133 Moby Thesaurus words for "fitness":
   Hygeia, ability, ableness, acceptability, account, adequacy,
   admissibility, advantage, advantageousness, advisability,
   applicability, appositeness, appropriateness, aptitude, aptness,
   auspiciousness, becomingness, beneficialness, bloom, caliber,
   capability, capableness, capacity, chargedness, civility,
   competence, competency, condition, convenience, correctitude,
   correctness, criticality, crucialness, decencies, decency,
   decorousness, decorum, desirability, efficacy, efficiency,
   eligibility, eligible, emotional health, expedience, expediency,
   facility, faculty, favorableness, feasibility, felicitousness,
   felicity, fettle, fine fettle, fittedness, fittingness, flair,
   flush, fruitfulness, genius, genteelness, gentility, glow,
   good shape, goodness, happiness, harmony, health, healthiness,
   kilter, loadedness, mastery, maturity, meetness, mental health,
   niceness, normality, normativeness, opportuneness, order,
   percentage, pertinence, physical condition, physical fitness,
   politicness, pregnancy, preparedness, proficiency, profit,
   profitability, properness, propitiousness, proprieties, propriety,
   prudence, qualification, readiness, relevance, repair,
   righteousness, rightness, ripeness, rosiness, salubriousness,
   salubrity, seasonableness, seasoning, seemliness, service,
   serviceability, shape, sufficiency, suitability, suitableness,
   suitedness, susceptibility, talent, tempering, the goods,
   the stuff, timeliness, tone, trim, urbanity, use, usefulness,
   utility, vigor, well-being, what it takes, wholesomeness, wisdom,
   worthiness, worthwhileness