The Collaborative International Dictionary of English v.0.48:

Intelligible \In*tel"li*gi*ble\, [L. intellegibilis: cf. F.
   intelligible. See Intelligent.]
   Capable of being understood or comprehended; as, an
   intelligible account or description; intelligible
   pronunciation, writing, etc.
   [1913 Webster]

         The intelligible forms of ancient poets. --Coleridge.

   Syn: Comprehensible; perspicuous; plain; clear.
        [1913 Webster]

WordNet (r) 3.0 (2006):

intelligible
    adj 1: capable of being apprehended or understood [syn:
           apprehensible, intelligible, graspable,
           perceivable, understandable]
    2: well articulated or enunciated, and loud enough to be heard
       distinctly; "intelligible pronunciation" [ant:
       unintelligible]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "intelligible":
   allegorical, apprehensible, articulate, associational, clear,
   cognizable, comprehensible, connotational, connotative,
   decipherable, definable, denotational, denotative,
   easily understood, easy to understand, exoteric, expressive,
   extended, extensional, fathomable, figurative, full of meaning,
   full of point, full of substance, graspable, indicative,
   intensional, interpretable, knowable, legible, lucid, luminous,
   meaning, meaningful, meaty, metaphorical, penetrable, pithy, plain,
   plumbable, pointed, pregnant, prehensible, readable, referential,
   scrutable, sententious, significant, significative, substantial,
   suggestive, symbolic, transferred, unambiguous, understandable