Aperture

n. opening; circular hole that controls the amount of light entering a camera

In optics, an aperture is a hole or an opening through which light is admitted. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. An optical system typically has many openings, or structures that limit the ray bundles (ray bundles are also known as pencils of light). These structures may be the edge of a lens or mirror, or a ring or other fixture that holds an optical element in place, or may be a special element such as a diaphragm placed in the optical path to limit the light admitted by the system. In general, these structures are called stops, and the aperture stop is the stop that determines the ray cone angle, or equivalently the brightness, at an image point.
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Noun
1. a device that controls amount of light admitted
(hypernym) regulator
(part-holonym) camera, photographic camera
2. a natural opening in something
(hypernym) hole
(hyponym) pupil
(part-holonym) eye, oculus, optic
3. an man-made opening; usually small
(hypernym) opening
(hyponym) mouthpiece, embouchure

n. opening; openness, aperture; hole, gap, gape, vent, slot, inlet, leak; span

v. open, move to an open position