noun (n)

• the relative frequency of occurrence of something(noun.linkdef)

  Synonym:
  relative incidence
  

  source: wordnet30
• the striking of a light beam on a surface(noun.phenomenon)

  Example:
  He measured the angle of incidence of the reflected light.
  

  source: wordnet30
• A falling on or upon; an incident; an event.(noun)

  source: webster1913