a device that controls amount of light admitted
"The photographer adjusted the aperture to let more light into the camera during the dim evening shoot."
a natural opening in something
"The sun streamed through the aperture left by the broken tree branch, illuminating the forest floor."
an man-made opening; usually small
"The photographer adjusted the lens aperture to let more light into the camera's small, man-made opening."
A small or narrow opening, gap, slit, or hole.
"The photographer adjusted the aperture to control how much light entered through the small circular opening in the lens."
In plain English: Aperture is the opening size that controls how much light enters through a lens.
"The photographer adjusted the aperture to get more light into the camera lens."
Usage: In photography and optics, aperture specifically refers to the adjustable opening in a lens that controls how much light enters the camera. While it can describe any physical gap like an iris pupil, everyday usage most often concerns this technical function rather than general openings.
The word aperture comes from the Late Middle English term for an "opening," which was borrowed directly from Latin apertūra. This Latin root combines a verb meaning to uncover or make bare with a suffix that forms nouns indicating an action, so it originally referred simply to something opened up.