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