aperture

An aperture is a crevice, hole, or separation. (noun)

An example of an aperture is the opening in a camera lense.

See aperture in Webster's New World College Dictionary

noun

an opening; hole; gap
the opening, or the diameter of the opening, in a camera, telescope, etc. through which light passes into the lens

See aperture in American Heritage Dictionary 4

(ăpˈər-chər)

noun

An opening, such as a hole, gap, or slit.
a. A usually adjustable opening in an optical instrument, such as a camera or telescope, that limits the amount of light passing through a lens or onto a mirror.
b. The diameter of such an opening, often expressed as an f-number.
c. The diameter of the objective of a telescope.

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