/ ˈæpətʃə(r); NAmE ; NAmE also -tʃʊr/ noun
1.
( formal ) a small opening in sth
2.
( technical ) an opening that allows light to reach a lens , especially in cameras :
For flash photography, set the aperture at f.5.6.
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WORD ORIGIN
late Middle English : from Latin apertura , from apert- opened, from aperire to open.