Main Entry: ap·er·ture

Pronunciation: \ˈap-ə(r)-ˌchu̇r, -chər, -ˌtyu̇r, -ˌtu̇r\

Function: noun

Etymology: Middle English, from Latin apertura, from apertus, past participle of aperire to open

Date: 15th century

1 : an opening or open space : hole
2 a : the opening in a photographic lens that admits the light b : the diameter of the stop in an optical system that determines the diameter of the bundle of rays traversing the instrument c : the diameter of the objective lens or mirror of a telescope