n.
Pronunciation: ' ap- ə (r)- ˌ chu ̇ r, -ch ə r, - ˌ tyu ̇ r, - ˌ tu ̇ r
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