The Collaborative International Dictionary of English v.0.48:

Intruder \In*trud"er\, n.
   1. One who intrudes; one who thrusts himself in, or enters
      without right, or without leave or welcome; a trespasser.
      [1913 Webster]

            They were all strangers and intruders. --Locke.
      [1913 Webster]

   2. Specifically: A person who enters a private residence or
      place of business with the intention to perform a criminal
      act; as, killed by an intruder.
      [PJC]

WordNet (r) 3.0 (2006):

intruder
    n 1: someone who intrudes on the privacy or property of another
         without permission [syn: intruder, interloper,
         trespasser]

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "intruder":
   arrival, arriviste, black sheep, blemish, burglar, busybody,
   buttinsky, comer, crasher, emigrant, encroacher, entrant,
   foreign body, foreign intruder, gate-crasher, gatecrasher,
   greenhorn, immigrant, impurity, in-migrant, incomer, infiltrator,
   interloper, invader, kibitzer, meddler, misfit, monkey wrench,
   mote, new arrival, new boy, newcomer, novus homo, oddball, parvenu,
   recruit, rookie, settler, sliver, snooper, speck, splinter,
   squatter, stone, stowaway, tenderfoot, thief, trespasser,
   uninvited guest, unwelcome guest, upstart, visitant, visitor,
   weed



Bouvier's Law Dictionary, Revised 6th Ed (1856):

INTRUDER. One who, on the death of the ancestor, enters on the land,
unlawfully, before the heir can enter.