/ ˈɪnsɪdəns; NAmE / noun
1.
[ C , usually sing. ] incidence of sth ( formal ) the extent to which sth happens or has an effect :
an area with a high incidence of crime
2.
[ U ] ( physics ) the way in which light meets a surface :
the angle of incidence
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WORD ORIGIN
late Middle English (denoting a casual or subordinate event): from Old French , or from medieval Latin incidentia , from Latin incidere fall upon, happen to, from in- upon + cadere to fall. Sense 1 dates from the early 19th cent.