An opening, hole, or gap
A stop that restricts the amount of light entering a lens system; a diaphragm with a circular hole that acts in this way.
An opening on the inner surface of a secondary cell wall, forming the entrance to a pit cavity.
The diameter of the widest beam of light incident on an optical system that goes to form an image; also in extended use (e.g. in connection with radio waves or particle beams).
A measure of the resolving power of a microscope objective (for a given wavelength), equal to the product of the refractive index of the medium in front of it and the sine of the semi-angle of the cone it subtends at the centre of the object.
The nasal aperture of the skull.
A measure of the light-collecting power of a lens, telescope, etc., which is usually expressed in terms of its f-number (a lower f-number corresponding to a larger aperture), and is used in determining the exposure needed.
A system used in some automatic cameras in which the aperture is selected by the user and the appropriate shutter speed is controlled automatically