The Collaborative International Dictionary of English v.0.48:

Comprehensible \Com"pre*hen"si*ble\, a. [L. comprehensibilis:
   cf. F. compre['e]hensible.]
   1. Capable of being comprehended, included, or comprised.
      [1913 Webster]

            Lest this part of knowledge should seem to any not
            comprehensible by axiom, we will set down some heads
            of it.                                --Bacon.
      [1913 Webster]

   2. Capable of being understood; intelligible; conceivable by
      the mind.
      [1913 Webster]

            The horizon sets the bounds . . . between what is
            and what is not comprehensible by us. --Locke.
      [1913 Webster]

WordNet (r) 3.0 (2006):

comprehensible
    adj 1: capable of being comprehended or understood; "an idea
           comprehensible to the average mind" [syn:
           comprehensible, comprehendible] [ant:
           incomprehensible, uncomprehensible]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "comprehensible":
   appreciable, apprehensible, articulate, ascertainable, cognizable,
   conceivable, discernible, discoverable, distinguishable,
   easily understood, easy to understand, exoteric, fathomable,
   graspable, intelligible, knowable, lucid, luminous, penetrable,
   perceptible, plumbable, prehensible, readable, recognizable,
   scrutable, seizable, understandable