Source : Webster's Revised Unabridged Dictionary (1913)

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

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

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

   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.

   4. (Scotch Ch.) The settlement of a minister over 3
      congregation without their consent.

Source : WordNet®

intrusion
     n 1: any entry into an area not previously occupied; "an invasion
          of tourists"; "an invasion of locusts" [syn: {invasion},
           {encroachment}]
     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}, {usurpation}]