Webster's English Dictionary

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incidence

in.ci.dence \'in(t)-s*d-*n(t)s, -s*-.den(t)s\ n 1a: an act or the fact or 
   manner of falling upon or affecting : OCCURR ENCE 1b: rate of occurrence or 
   influence  2a: the arrival of something (as a projectile or a ray of light) 
   at a surfa ce 2b: ANGLE OF INCIDENCE