Source : Webster's Revised Unabridged Dictionary (1913)

Intruder \In*trud"er\, n.
   One who intrudes; one who thrusts himself in, or enters
   without right, or without leave or welcome; a trespasser.

         They were all strangers and intruders.   --Locke.

Source : WordNet®

intruder
     n : someone who intrudes on the privacy or property of another
         without permission [syn: {interloper}, {trespasser}]