Source : Webster's Revised Unabridged Dictionary (1913)

Thickness \Thick"ness\, n. [AS. ?icnes.]
   The quality or state of being thick (in any of the senses of
   the adjective).

Source : WordNet®

thickness
     n 1: the dimension through an object as opposed to its length or
          width [ant: {thinness}]
     2: used of a line or mark [syn: {heaviness}]
     3: resistance to flow [ant: {thinness}]