Source : Webster's Revised Unabridged Dictionary (1913)

Diffuseness \Dif*fuse"ness\, n.
   The quality of being diffuse; especially, in writing, the use
   of a great or excessive number of word to express the
   meaning; copiousness; verbosity; prolixity.

Source : WordNet®

diffuseness
     n : the spatial property of being spread out over a wide area or
         through a large volume