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]

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "intrusion":
   access, admission, alienage, alienism, encroachment, entrance,
   entree, entry, exteriority, extraneousness, extrinsicality,
   foreignness, import, importation, importing, impropriety,
   inappropriateness, inauspiciousness, income, incoming,
   inconvenience, incursion, inexpedience, infelicity, infiltration,
   infraction, infringement, ingoing, ingress, ingression,
   inopportuneness, inopportunity, input, inroad, insertion,
   insinuation, intake, intempestivity, intercurrence, interjacence,
   interlocation, intermediacy, interpenetration, interposition,
   interposure, interruption, intervenience, intervention,
   introduction, introgression, irrelevance, lateness, leakage,
   nonassimilation, nonconformity, overstepping, penetration,
   percolation, prematurity, reception, sandwiching, seepage,
   transgression, trespass, unfavorableness, unfitness, unfittingness,
   unfortunateness, unpropitiousness, unripeness, unseasonableness,
   unsuitability, untimeliness, usurpation, wrongness



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

INTRUSION, remedies. The name of a writ, brought by the owner of a fee
simple, &c., against an intruder. New Nat. Br. 453.



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

INTRUSION, estates, torts. When an ancestor dies seised of an estate of
inheritance expectant upon an estate for life, and then the tenant dies, and
between his death and the entry of the heir, a stranger unlawfully enters
upon the estate, this is called an intrusion. It differs from an abatement,
for the latter is an entry into lands void by the death of a tenant in fee,
and an intrusion, as already stated, is an entry into land void by the death
of a tenant for years. F. N. B. 203 3 Bl. Com. 169 Archb. Civ. Pl. 12;
Dane's Ab. Index, h. t.