noun (n)

• a sudden violent entrance; a bursting in(noun.act)

  Example:
  The recent irruption of bad manners.
  

  source: wordnet30
• a sudden sharp increase in the relative numbers of a population(noun.process)

  source: wordnet30
• a sudden violent spontaneous occurrence (usually of some undesirable condition)(noun.event)

  Synonym:
  eruption, outbreak
  

  source: wordnet30
• A bursting in; a sudden, violent rushing into a place; as, irruptions of the sea.(noun)

  source: webster1913