The Collaborative International Dictionary of English v.0.48:

Intrusion \In*tru"sion\, n. [Cf. F. intrusion. See Intrude.]
   [1913 Webster]
   1. The act of intruding, or of forcing in; especially, the
      forcing (one's self) into a place without right or
      welcome; encroachment.
      [1913 Webster]

            Why this intrusion?
            Were not my orders that I should be private?
                                                  --Addison.
      [1913 Webster]

   2. (Geol.) The penetrating of one rock, while in a plastic or
      metal state, into the cavities of another.
      [1913 Webster]

   3. (Law) The entry of a stranger, after a particular estate
      or freehold is determined, before the person who holds in
      remainder or reversion has taken possession.
      [1913 Webster]

   4. (Scotch Ch.) The settlement of a minister over a
      congregation without their consent.
      [1913 Webster]

WordNet (r) 3.0 (2006):

intrusion
    n 1: any entry into an area not previously occupied; "an
         invasion of tourists"; "an invasion of locusts" [syn:
         invasion, encroachment, intrusion]
    2: entrance by force or without permission or welcome
    3: the forcing of molten rock into fissures or between strata of
       an earlier rock formation
    4: rock produced by an intrusive process
    5: entry to another's property without right or permission [syn:
       trespass, encroachment, violation, intrusion,
       usurpation]