The Collaborative International Dictionary of English v.0.48:

Interdependency \In`ter*de*pend"en*cy\, n.
   Mutual dependence; as, interdependency of interests. --De
   Quincey.
   [1913 Webster]

WordNet (r) 3.0 (2006):

interdependency
    n 1: a reciprocal relation between interdependent entities
         (objects or individuals or groups) [syn: mutuality,
         interdependence, interdependency]