The Collaborative International Dictionary of English v.0.48:

externalise \externalise\ v.
   to make external.

   Syn: exteriorize, externalize.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

externalise
    v 1: regard as objective [syn: project, externalize,
         externalise]
    2: make external or objective, or give reality to; "language
       externalizes our thoughts" [syn: exteriorize,
       exteriorise, externalize, externalise, objectify]