Wordnet 3.0
ADJECTIVE (2)
1. 
 subject to alteration or change; 
- Example: "the death sentence was commutable to life imprisonment"2. 
 capable of being exchanged for another or for something else that is equivalent; 
[syn: commutable, substitutable]
The Collaborative International Dictionary of English v.0.48:
Commutable \Com*mut"a*ble\, a. [L. commutabilis.]
   Capable of being commuted or interchanged.
   [1913 Webster]
         The predicate and subject are not commutable.
                                                  --Whately.
   [1913 Webster]
WordNet (r) 3.0 (2006):
commutable
    adj 1: subject to alteration or change; "the death sentence was
           commutable to life imprisonment" [ant: incommutable]
    2: capable of being exchanged for another or for something else
       that is equivalent [syn: commutable, substitutable]