The Collaborative International Dictionary of English v.0.48:

Otherness \Oth"er*ness\, n.
   The quality or state of being other or different; alterity;
   oppositeness.
   [1913 Webster]

WordNet (r) 3.0 (2006):

otherness
    n 1: the quality of being not alike; being distinct or different
         from that otherwise experienced or known [syn: otherness,
         distinctness, separateness]