The Collaborative International Dictionary of English v.0.48:

Invasion \In*va"sion\, n. [L. invasio: cf. F. invasion. See
   Invade.]
   [1913 Webster]
   1. The act of invading; the act of encroaching upon the
      rights or possessions of another; encroachment; trespass.
      [1913 Webster]

   2. A warlike or hostile entrance into the possessions or
      domains of another; the incursion of an army for conquest
      or plunder.
      [1913 Webster]

   3. The incoming or first attack of anything hurtful or
      pernicious; as, the invasion of a disease.

   Syn: Invasion, Irruption, Inroad.

   Usage: Invasion is the generic term, denoting a forcible
          entrance into a foreign country. Incursion signifies a
          hasty and sudden invasion. Irruption denotes
          particularly violent invasion. Inroad is entry by some
          unusual way involving trespass and injury.
          [1913 Webster]

WordNet (r) 3.0 (2006):

invasion
    n 1: the act of invading; the act of an army that invades for
         conquest or plunder
    2: any entry into an area not previously occupied; "an invasion
       of tourists"; "an invasion of locusts" [syn: invasion,
       encroachment, intrusion]
    3: (pathology) the spread of pathogenic microorganisms or
       malignant cells to new sites in the body; "the tumor's
       invasion of surrounding structures"