1. an opening, as a hole, slit, crack, gap, etc. 2. Also called aperture stop. Optics. an opening, usually circular, that limits the quantity of light that can enter an optical instrument.
He remained motionless and pensive, his eyes fixed on the gloomy aperture that was open at his feet. — Alexandre Dumas, The Count of Monte Cristo, 1844
He made for it a large aperture with a cover pierced with numerous holes, and, taking forth some new nails, said to the young lion, Enter… The young lion therefore rejoiced at this, and advanced to the aperture; but he saw that it was narrow. — Edward William Lane and Edward Stanley Poole, The Thousand and One Nights, 1865
Aperture entered English in the mid-1600s from the Latin aperīre meaning “to open.”