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):

perspicuous
    adj 1: (of language) transparently clear; easily understandable;
           "writes in a limpid style"; "lucid directions"; "a
           luculent oration"- Robert Burton; "pellucid prose"; "a
           crystal clear explanation"; "a perspicuous argument"
           [syn: limpid, lucid, luculent, pellucid, crystal
           clear, perspicuous]

Moby Thesaurus II by Grady Ward, 1.0:

105 Moby Thesaurus words for "perspicuous":
   Attic, Ciceronian, acute, apparent, apperceptive, appercipient,
   argute, astute, chaste, classic, clean-cut, clear,
   clear as crystal, clear as day, clear-cut, cogent, coherent,
   connected, consistent, crisp, crystal, crystal-clear, crystalline,
   defined, definite, direct, discernible, discerning, distinct, easy,
   elegant, evident, explicit, express, farseeing, farsighted,
   finished, foreseeing, foresighted, forethoughted, forethoughtful,
   graceful, gracile, incisive, indisputable, indubitable, limpid,
   longheaded, longsighted, loud and clear, lucent, lucid, luculent,
   luminous, manifest, natural, neat, noticeable, observable, obvious,
   open-and-shut, palpable, patent, pellucid, penetrating,
   perceivable, perceptible, perceptive, percipient, perspicacious,
   piercing, plain, plain as day, polished, provident, pure, refined,
   restrained, round, sagacious, seeable, self-evident,
   self-explaining, self-explanatory, simple, straightforward,
   tangible, tasteful, terse, to be seen, translucent, transparent,
   transpicuous, trenchant, trim, unaffected, unambiguous, unconfused,
   understanding, unequivocal, univocal, unlabored, unmistakable,
   visible, well-defined