The Collaborative International Dictionary of English v.0.48:

Understandable \Un`der*stand"a*ble\
   ([u^]n`d[~e]r*st[a^]nd"[.a]*b'l), a.
   Capable of being understood; intelligible. --Chillingworth.
   [1913 Webster]

WordNet (r) 3.0 (2006):

understandable
    adj 1: capable of being apprehended or understood [syn:
           apprehensible, intelligible, graspable,
           perceivable, understandable]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "understandable":
   appreciable, apprehensible, articulate, ascertainable, clear-cut,
   cognizable, comprehensible, conceivable, discernible, discoverable,
   distinguishable, easily understood, easy to understand, exoteric,
   fathomable, graspable, intelligible, knowable, lay, lucid,
   luminous, penetrable, perceptible, plain, plumbable, popular,
   prehensible, readable, recognizable, scrutable, seizable, simple,
   straightforward, unambiguous