The Collaborative International Dictionary of English v.0.48:

Invader \In*vad"er\, n.
   One who invades; an assailant; an encroacher; an intruder.
   [1913 Webster]

WordNet (r) 3.0 (2006):

invader
    n 1: someone who enters by force in order to conquer [syn:
         invader, encroacher]