Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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: {tenuity}, {slenderness}]
          [ant: {thickness}]
     2: the property of having little body fat [syn: {leanness}]
        [ant: {fatness}]
     3: the property of being very narrow or thin; "he marvelled at
        the fineness of her hair" [syn: {fineness}]
     4: a consistency of low viscosity; "he disliked the thinness of
        the soup" [ant: {thickness}]