noun (n)

• the act of invading; the act of an army that invades for conquest or plunder(noun.act)

  source: wordnet30
• any entry into an area not previously occupied(noun.event)

  Example:
  An invasion of tourists.
  An invasion of locusts.
  

  Synonym:
  encroachment, intrusion
  

  source: wordnet30
• (pathology) the spread of pathogenic microorganisms or malignant cells to new sites in the body(noun.event)

  Example:
  The tumor's invasion of surrounding structures.
  

  source: wordnet30
• The act of invading; the act of encroaching upon the rights or possessions of another; encroachment; trespass.(noun)

  source: webster1913