|1.||a hole, gap, crack, slit, or other opening|
|a. a usually circular and often variable opening in an optical instrument or device that controls the quantity of radiation entering or leaving it|
|b. See also relative aperture the diameter of such an opening|
|[C15: from Late Latin apertūra opening, from Latin aperīre to open]|
aperture ap·er·ture (āp'ər-chər)
An opening, such as a hole, gap, or slit.
A usually adjustable opening in an optical instrument, such as a microscope, a camera, or a telescope, that limits the amount of light passing through a lens or onto a mirror.
The diameter of such an opening.
The diameter of the objective of a telescope or microscope.