The Collaborative International Dictionary of English v.0.48:

Permutable \Per*mut"a*ble\ (p[~e]r*m[=u]"t[.a]*b'l), a. [Cf. F.
   permutable.]
   Capable of being permuted; exchangeable. --
   Per*mut"a*ble*ness, n. -- Per*mut"a*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

permutable
    adj 1: capable of changing sequence [syn: transposable,
           permutable]