From WordNet (r) 2.0 (August 2003) [wn]:
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]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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.
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2. (Geol.) The penetrating of one rock, while in a plastic or
metal state, into the cavities of another.
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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]