The Collaborative International Dictionary of English v.0.48:

Simplicity \Sim*plic"i*ty\, n. [F. simplicit['e], L.
   simplicitas. See Simple.]
   1. The quality or state of being simple, unmixed, or
      uncompounded; as, the simplicity of metals or of earths.
      [1913 Webster]

   2. The quality or state of being not complex, or of
      consisting of few parts; as, the simplicity of a machine.
      [1913 Webster]

   3. Artlessness of mind; freedom from cunning or duplicity;
      lack of acuteness and sagacity.
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            Marquis Dorset, a man, for his harmless simplicity
            neither misliked nor much regarded.   --Hayward.
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            In wit a man; simplicity a child.     --Pope.
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   4. Freedom from artificial ornament, pretentious style, or
      luxury; plainness; as, simplicity of dress, of style, or
      of language; simplicity of diet; simplicity of life.
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   5. Freedom from subtlety or abstruseness; clearness; as, the
      simplicity of a doctrine; the simplicity of an explanation
      or a demonstration.
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   6. Weakness of intellect; silliness; folly.
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            How long, ye simple ones, will ye love simplicity?
            and the scorners delight in their scorning? --Prov.
                                                  i. 22.
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WordNet (r) 3.0 (2006):

simplicity
    n 1: the quality of being simple or uncompounded; "the
         simplicity of a crystal" [syn: simplicity, simpleness]
         [ant: complexity, complexness]
    2: a lack of penetration or subtlety; "they took advantage of
       her simplicity" [syn: simplicity, simpleness, simple
       mindedness]
    3: absence of affectation or pretense [syn: simplicity,
       simmpleness]
    4: freedom from difficulty or hardship or effort; "he rose
       through the ranks with apparent ease"; "they put it into
       containers for ease of transportation"; "the very easiness of
       the deed held her back" [syn: ease, easiness,
       simplicity, simpleness] [ant: difficultness,
       difficulty]
    5: lack of ornamentation; "the room was simply decorated with
       great restraint" [syn: chasteness, restraint,
       simplicity, simpleness]

Moby Thesaurus II by Grady Ward, 1.0:

317 Moby Thesaurus words for "simplicity":
   Attic simplicity, Atticism, Bohemianism, abatement, abridgment,
   affability, agnosticism, agrarianism, alleviation, amentia,
   appropriateness, arrested development, artlessness, asceticism,
   attenuation, austereness, austerity, backwardness, baldness,
   bareness, blankmindedness, blithering idiocy, bluffness, bluntness,
   boorishness, brainlessness, bucolicism, callowness, candor,
   casualness, chasteness, chastity, childlikeness, churlishness,
   clarity, classical purity, classicalism, classicism, cleanness,
   clear coast, clear road, clear-cutness, clearness, cloddishness,
   coherence, combination, comeliness, common speech, common touch,
   commonness, commonplaceness, comprehensibility, connectedness,
   consistency, contraction, cordiality, correctness, cretinism,
   crystallinity, cullibility, dampening, damping, deceivability,
   decipherability, decrease, decrement, decrescence, deduction,
   definition, deflation, depreciation, depression, dignity,
   diminishment, diminution, directness, discrimination, distinction,
   distinctness, dull-wittedness, dullness, dupability, dying,
   dying off, ease, easiness, easy going, easygoingness,
   effortlessness, elegance, elegancy, empty-headedness, explicitness,
   extenuation, facileness, facility, fade-out, familiarity,
   felicitousness, felicity, finish, fittingness, flow,
   flowing periods, fluency, folksiness, forthrightness, frankness,
   freedom, fusion, good taste, grace, gracefulness, gracility,
   graciousness, greenhornism, greenness, guilelessness, gullibility,
   half-wittedness, hiatus of learning, highroad, hoaxability,
   homeliness, homespun, homeyness, household words, identity, idiocy,
   idiotism, ignorance, ignorantism, ignorantness, imbecility,
   inanity, individuality, indivisibility, inelegance, inexperience,
   infantilism, informality, informalness, ingenuousness, innocence,
   insanity, intactness, integrality, integration, integrity,
   intelligibility, inurbanity, inviolability, irreducibility,
   irregularity, know-nothingism, knowledge-gap, lack of hindrance,
   lack of information, languishment, leanness, lessening, letup,
   limpidity, looseness, lowering, lucidity, matter-of-factness,
   mental defectiveness, mental deficiency, mental handicap,
   mental retardation, miniaturization, mitigation, modesty,
   mongolianism, mongolism, mongoloid idiocy, moronism, moronity,
   naiveness, naivete, naivety, naturalness, neatness, nescience,
   oafishness, obscurantism, obtuseness, offhandedness, oneness,
   openheartedness, openness, ordinariness, organic unity,
   outspokenness, particularity, pastorality, pellucidity,
   perspicuity, persuadability, plain English, plain sailing,
   plain speaking, plain speech, plain style, plain words, plainness,
   plane sailing, polish, profound idiocy, propriety, prosaicness,
   prosiness, provincialism, provinciality, purity, quietness,
   rawness, reduction, refinement, relaxation, relaxedness,
   restrainedness, restraint, retardation, retardment, royal road,
   ruralism, rustic style, rusticity, sagging, scaling down,
   seduceability, seemliness, selfsameness, severity,
   simple-wittedness, simpleheartedness, simplemindedness, simpleness,
   sincerity, single-heartedness, single-mindedness, singleness,
   singleness of heart, singularity, smooth road, smooth sailing,
   smoothness, soberness, sociability, softness, solidarity,
   solidification, solidity, spareness, starkness, straight sailing,
   straightforwardness, structure, stupidity, subduedness,
   subnormality, subtraction, tabula rasa, taste, tastefulness,
   terseness, transparency, transpicuity, trustfulness,
   unacquaintance, unadorned style, unadornedness, unaffectedness,
   unambiguousness, unceremoniousness, uncomplexity,
   uncomplicatedness, unconstrainedness, unconstraint,
   unconventionality, uncultivation, understandability,
   understatement, undividedness, unequivocalness, unfamiliarity,
   unguardedness, unification, uniformity, unimaginativeness,
   unintelligence, uniqueness, unity, univocity, unknowing,
   unknowingness, unmistakableness, unobtrusiveness, unpoeticalness,
   unpretentiousness, unrefinement, unripeness, unsophisticatedness,
   unsophistication, unspoiledness, unsuspiciousness, unwariness,
   vacuity, vacuousness, vernacular, weakening, weakness, wholeness,
   witlessness, yokelism