The Collaborative International Dictionary of English v.0.48:

Perspicuous \Per*spic"u*ous\, a. [L. perspicuus, from perspicere
   to look through. See Perspective.]
   1. Capable of being through; transparent; translucent; not
      opaque. [Obs.] --Peacham.
      [1913 Webster]

   2. Clear to the understanding; capable of being clearly
      understood; clear in thought or in expression; not obscure
      or ambiguous; as, a perspicuous writer; perspicuous
      statements. "The purpose is perspicuous." --Shak.
      [1913 Webster] -- Per*spic"u*ous*ly, adv. --
      Per*spic"u*ous*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

perspicuousness
    n 1: clarity as a consequence of being perspicuous [syn:
         perspicuity, perspicuousness, plainness]