The Collaborative International Dictionary of English v.0.48:

Coextensive \Co`ex*ten"sive\, a.
   Equally extensive; having equal extent; as, consciousness and
   knowledge are coextensive. --Sir W. Hamilton. --
   Co`ex*ten"sive*ly, adv. -- Co`ex*ten"sive*ness, n.
   [1913 Webster]