Source : Webster's Revised Unabridged Dictionary (1913)

Understandable \Un`der*stand"a*ble\, a.
   Capable of being understood; intelligible. --Chillingworth.

Source : WordNet®

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