The Collaborative International Dictionary of English v.0.48:

Sincerity \Sin*cer"i*ty\, n. [L. sinceritas: cf. F.
   sinc['e]rit['e].]
   The quality or state of being sincere; honesty of mind or
   intention; freedom from simulation, hypocrisy, disguise, or
   false pretense; sincereness.
   [1913 Webster]

         I protest, in the sincerity of love.     --Shak.
   [1913 Webster]

         Sincerity is a duty no less plain than important.
                                                  --Knox.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sincerity
    n 1: an earnest and sincere feeling [syn: earnestness,
         seriousness, sincerity]
    2: the quality of being open and truthful; not deceitful or
       hypocritical; "his sincerity inspired belief"; "they demanded
       some proof of my sincerity" [ant: falseness, hollowness,
       insincerity]
    3: a quality of naturalness and simplicity; "the simple
       sincerity of folk songs" [syn: sincerity, unassumingness]
    4: the trait of being serious; "a lack of solemnity is not
       necessarily a lack of seriousness"- Robert Rice [syn:
       seriousness, earnestness, serious-mindedness,
       sincerity] [ant: frivolity, frivolousness]

Moby Thesaurus II by Grady Ward, 1.0:

123 Moby Thesaurus words for "sincerity":
   abandon, absolute realism, ardency, ardor, artlessness,
   authenticity, bluffness, bluntness, bona fideness, broadness,
   brusqueness, candidness, candor, childlikeness, commitment,
   committedness, decidedness, decision, decisiveness, dedication,
   definiteness, determinateness, determination, determinedness,
   devotedness, devotion, devoutness, directness, earnestness, faith,
   faithfulness, fervency, fervidness, fervor, fidelity, fire,
   forthrightness, frankness, freedom, freeness, genuineness,
   guilelessness, heartiness, heat, heatedness, honesty,
   impassionedness, inartificiality, ingenuousness, innocence,
   intensity, intentness, legitimacy, lifelikeness, literalism,
   literality, literalness, loyalty, naiveness, naivete, naivety,
   naturalism, naturalness, obstinacy, openheartedness, openness,
   outspokenness, passion, passionateness, perseverance, persistence,
   photographic realism, plain dealing, plain speaking, plainness,
   plainspokenness, purpose, purposefulness, realism, realness,
   relentlessness, resoluteness, resolution, resolve, resolvedness,
   roundness, self-will, seriousness, simpleheartedness,
   simplemindedness, simpleness, simplicity, single-heartedness,
   single-mindedness, singleness of heart, spirit,
   straightforwardness, tenacity, total commitment, true-to-lifeness,
   trustfulness, truth to nature, truthfulness, unadulteration,
   unaffectedness, unconstraint, unfictitiousness, unguardedness,
   unreserve, unrestraint, unsophisticatedness, unsophistication,
   unspeciousness, unspuriousness, unsuspiciousness, unsyntheticness,
   unwariness, uprightness, vehemence, verisimilitude, warmth, will,
   zeal