The Collaborative International Dictionary of English v.0.48:

Externality \Ex`ter*nal"i*ty\, n.
   State of being external; exteriority; (Metaph.) separation
   from the perceiving mind.
   [1913 Webster]

         Pressure or resistance necessarily supposes externality
         in the thing which presses or resists.   --A. Smith.
   [1913 Webster]

WordNet (r) 3.0 (2006):

externality
    n 1: the quality or state of being outside or directed toward or
         relating to the outside or exterior; "the outwardness of
         the world" [syn: outwardness, externality] [ant:
         inwardness]