Source : Webster's Revised Unabridged Dictionary (1913)

Alterable \Al"ter*a*ble\, a. [Cf. F. alt['e]rable.]
   Capable of being altered.

         Our condition in this world is mutable and uncertain,
         alterable by a thousand accidents.       --Rogers.

Source : WordNet®

alterable
     adj 1: capable of being changed or altered in some characteristic;
            "alterable clothing"; "alterable conditions of
            employment" [ant: {unalterable}]
     2: of a sentence; capable of being changed to one less severe