The Collaborative International Dictionary of English v.0.48:

Coextension \Co`ex*ten"sion\ (k[=o]`[e^]ks*t[e^]n"sh[u^]n), n.
   The act of extending equally, or the state of being equally
   extended.
   [1913 Webster]

WordNet (r) 3.0 (2006):

coextension
    n 1: equality of extension or duration