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]