n 1 a  :  ANGLE OF INCIDENCE  b  :  the arrival of something (as a ray of light) at a surface  2 a  :  an act or the fact or manner of occurring or affecting <diseases of domestic -Science>   b  :  rate of occurrence or influence : esp  :  the rate of occurrence of new cases of a particular disease in a population being studied - compare PREVALENCE  .
 
   
 
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