Wordnet 3.0
ADJECTIVE (2)
1. 
 capable of being changed or altered in some characteristic; 
- Example: "alterable clothing"- Example: "alterable conditions of employment"2. 
 (of the punishment ordered by a court) capable of being changed to one less severe; 
The Collaborative International Dictionary of English v.0.48:
Alterable \Al"ter*a*ble\, a. [Cf. F. alt['e]rable.]
   Capable of being altered.
   [1913 Webster]
         Our condition in this world is mutable and uncertain,
         alterable by a thousand accidents.       --Rogers.
   [1913 Webster]
WordNet (r) 3.0 (2006):
alterable
    adj 1: capable of being changed or altered in some
           characteristic; "alterable clothing"; "alterable
           conditions of employment" [ant: inalterable,
           unalterable]
    2: (of the punishment ordered by a court) capable of being
       changed to one less severe