The Collaborative International Dictionary of English v.0.48:

Thinness \Thin"ness\, n.
   The quality or state of being thin (in any of the senses of
   the word).
   [1913 Webster]

WordNet (r) 3.0 (2006):

thinness
    n 1: relatively small dimension through an object as opposed to
         its length or width; "the tenuity of a hair"; "the thinness
         of a rope" [syn: thinness, tenuity, slenderness]
         [ant: thickness]
    2: the property of having little body fat [syn: leanness,
       thinness, spareness] [ant: avoirdupois, blubber,
       fat, fatness]
    3: the property of being very narrow or thin; "he marvelled at
       the fineness of her hair" [syn: fineness, thinness]
    4: the property of being scanty or scattered; lacking denseness
       [syn: sparseness, spareness, sparsity, thinness]
    5: a consistency of low viscosity; "he disliked the thinness of
       the soup" [ant: thickness]

Moby Thesaurus II by Grady Ward, 1.0:

124 Moby Thesaurus words for "thinness":
   Lenten fare, admission of light, airiness, airy nothing,
   airy texture, attenuation, austerity, bodilessness, chinchiness,
   chintziness, clarity, clearness, crystal-clearness, crystallinity,
   deadness, delicacy, diaphaneity, diaphanousness, dilutedness,
   dilution, ethereality, exiguity, exiguousness, exility, fewness,
   filminess, fineness, flatness, flavorlessness, flightiness,
   flimsiness, fluffiness, frailty, frivolousness, frothiness,
   gauziness, glassiness, glasslikeness, gossameriness, gracility,
   immateriality, inanity, incorporeality, infrequency, insipidity,
   insipidness, insolidity, insubstantiality, jejuneness, jejunity,
   laciness, lack of depth, leanness, lifelessness, lightness,
   limpidity, lucidity, meagerness, meanness, mildness, miserliness,
   mistiness, narrowness, niggardliness, nonopacity, paltriness,
   paperiness, parsimony, paucity, pellucidity, puniness, rareness,
   rarity, restrictedness, saplessness, savorlessness, scantiness,
   scantness, scarcity, scrawniness, scrimpiness, shallow-wittedness,
   shallowness, sheerness, show-through, skimpiness, slenderness,
   slightness, slim pickings, slimness, smallness, spareness,
   sparseness, sparsity, staleness, stinginess, stringency, subtility,
   subtilty, subtlety, superficiality, tastelessness, tenuity,
   tenuousness, tightness, transmission of light, transparence,
   transparency, transpicuousness, uncloudedness, unprofoundness,
   unprofundity, unsavoriness, unsubstantiality, vagueness, vapidity,
   vitreosity, vitreousness, vitrescence, volatility, wateriness,
   weakness, wishy-washiness, wispiness