Source : Webster's Revised Unabridged Dictionary (1913)

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

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

Source : WordNet®

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