The Collaborative International Dictionary of English v.0.48:

Pithiness \Pith"i*ness\, n.
   The quality or state of being pithy.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pithiness
    n 1: terseness and economy in writing and speaking achieved by
         expressing a great deal in just a few words [syn:
         conciseness, concision, pithiness, succinctness]