1. an opening, as a hole, slit, crack, gap, etc.
  2. Also called aperture stop. Optics. an opening, usually circular, that limits the quantity of light that can enter an optical instrument.


  1. a hole, gap, crack, slit, or other opening
  2. physics
    1. a usually circular and often variable opening in an optical instrument or device that controls the quantity of radiation entering or leaving it
    2. the diameter of such an openingSee also relative aperture

early 15c., from Latin apertura “an opening,” from apertus, past participle of aperire “to open” (see overt).


  1. An opening, such as a hole, gap, or slit.
  2. 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.
  3. The diameter of such an opening.
  4. The diameter of the objective of a telescope or microscope.

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