aperture

[ap-er-cher]
noun
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.

Origin:
1400–50; late Middle English < Latin apertūra an opening, equivalent to apert(us) opened (past participle of aperīre; aper(i)- (see aperient) + -tus past participle suffix) + -ūra -ure

apertural [ap-er-choor-uhl] , adjective
apertured, adjective
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World English Dictionary
aperture (ˈæpətʃə)
 
n
1.  a hole, gap, crack, slit, or other opening
2.  physics
 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]

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Etymonline
Word Origin & History

aperture
1640s, from L. apertura "an opening," from apertus, pp. of aperire "to open" (see overt).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

aperture ap·er·ture (āp'ər-chər)
n.

  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.


ap'er·tur'al adj.
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Example sentences
You'll get much better low-light shots with larger apertures.
They asked volunteers to describe how they perceived the motion of moving lines
  seen through apertures.
To measure a star's diameter, one increases the separation between the
  apertures until the fringes disappear.
It measured the light striking it and suggested a range of appropriate
  combinations of apertures and shutter speeds.
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