Source : Webster's Revised Unabridged Dictionary (1913)

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

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

   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.

Source : WordNet®

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: {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"