From The Collaborative International Dictionary of English v.0.48 :
indistinguishability \in`dis*tin`guish*a*bil"i*ty\ n.
smilarity too close to be distinguished; exact sameness.
Syn: identity, identicalness.
[WordNet 1.5]
From WordNet (r) 3.0 (2006) :
indistinguishability
n 1: exact sameness; "they shared an identity of interests"
[syn: identity, identicalness, indistinguishability]