The Collaborative International Dictionary of English v.0.48:

Perspicuity \Per`spi*cu"i*ty\ (p[~e]r`sp[i^]*k[=u]"[i^]*t[y^]),
   n. [L. perspicuitas: cf. F. perspicuit['e].]
   1. The quality or state of being transparent or translucent.
      [Obs.] --Sir T. Browne.
      [1913 Webster]

   2. The quality of being perspicuous to the understanding;
      clearness of expression or thought.
      [1913 Webster]

   3. Sagacity; perspicacity.
      [1913 Webster]

   Syn: Clearness; perspicuousness; plainness; distinctness;
        lucidity; transparency. See Clearness.
        [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

143 Moby Thesaurus words for "perspicuity":
   Atticism, acuity, acumen, acuteness, apparentness, apperception,
   appropriateness, astuteness, chasteness, chastity, clarity,
   classicalism, classicism, clear sight, clear-cutness, clearness,
   cogency, coherence, color vision, comeliness, cone vision,
   connectedness, consistency, correctness, crystal-clearness,
   crystallinity, day vision, daylight vision, definition, dignity,
   directness, discernment, discrimination, distinction, distinctness,
   ease, elegance, elegancy, evidence, evidentness, explicitness, eye,
   eye-mindedness, eyesight, farseeingness, farsight, farsightedness,
   felicitousness, felicity, field of view, field of vision, finish,
   fittingness, flow, flowing periods, fluency, foresight,
   foresightedness, good taste, grace, gracefulness, gracility,
   horizon, incisiveness, insight, intelligibility, keen sight, ken,
   limpidity, longheadedness, longsightedness, lucidity, manifestness,
   naturalness, neatness, night vision, obviousness, openness,
   openness to sight, overtness, palpability, patentness, pellucidity,
   penetration, perception, perceptiveness, percipience,
   peripheral field, peripheral vision, perspicaciousness,
   perspicacity, perspicuousness, photopia, plain English,
   plain speech, plain style, plainness, polish, power of sight,
   propriety, providence, purity, purview, quick sight, range,
   refinement, restraint, rod vision, sagaciousness, sagacity, scope,
   scotopia, seeing, seemliness, sense of sight, sensibility, sight,
   sightedness, simplicity, smoothness, straightforwardness,
   structure, sweep, tangibility, taste, tastefulness, terseness,
   transparency, transpicuity, trenchancy, twilight vision,
   unadorned style, unaffectedness, unambiguousness, unequivocalness,
   unmistakableness, unobstructed vision, unquestionability,
   visibility, vision, visual acuity, visual field, visual sense