The Collaborative International Dictionary of English v.0.48:
Alterability \Al`ter*a*bil"i*ty\, n. [Cf. F. alt['e]rabilit['e].] The quality of being alterable; alterableness. [1913 Webster]WordNet (r) 3.0 (2006):
alterability n 1: the quality of being alterable [ant: unalterability]