The Collaborative International Dictionary of English v.0.48:

Encroachment \En*croach"ment\, n.
   1. The act of entering gradually or silently upon the rights
      or possessions of another; unlawful intrusion.
      [1913 Webster]

            An unconstitutional encroachment of military power
            on the civil establishment.           --Bancroft.
      [1913 Webster]

   2. That which is taken by encroaching on another.
      [1913 Webster]

   3. (Law) An unlawful diminution of the possessions of
      another.
      [1913 Webster]

WordNet (r) 3.0 (2006):

encroachment
    n 1: any entry into an area not previously occupied; "an
         invasion of tourists"; "an invasion of locusts" [syn:
         invasion, encroachment, intrusion]
    2: entry to another's property without right or permission [syn:
       trespass, encroachment, violation, intrusion,
       usurpation]
    3: influencing strongly; "they resented the impingement of
       American values on European culture" [syn: impingement,
       encroachment, impact]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "encroachment":
   adoption, appropriation, arrogation, assumption, entrance,
   entrenchment, impingement, imposition, incursion, infiltration,
   influx, infraction, infringement, injection, inroad, insinuation,
   interference, interjection, interloping, interposition,
   interposure, interruption, intervention, intrusion, invasion,
   irruption, obtrusion, overstepping, playing God, seizure,
   transgression, trespass, trespassing, unlawful entry, usurpation



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

ENCROACHMENT. An unlawful gaining upon the right or possession of another;
as, when a man sets his fence beyond his line; in this case the proper
remedy for the party injured is an action of ejectment, or an action of
trespass.