Source : Webster's Revised Unabridged Dictionary (1913)

Heaviness \Heav"i*ness\, n.
   The state or quality of being heavy in its various senses;
   weight; sadness; sluggishness; oppression; thickness.

Source : WordNet®

heaviness
     n 1: the property of being comparatively great in weight; "the
          heaviness of lead" [syn: {weightiness}] [ant: {lightness}]
     2: used of a line or mark [syn: {thickness}]