Definition of "aperture"

aperture

/ˈap.ə.t͡ʃə/

noun
plural apertures
1. A small or narrow opening, gap, slit, or hole.
  Quotations
  Were the larynx of the little creature like that of the parent, the milk might—probably would—enter the windpipe and cause suffocation : but the fœtal larynx is cone-shaped, with the opening at the apex, which projects, as in the whale-tribe, into the back aperture of the nostrils, where it is closely embraced by muscles of the ‘soft palate.’
  1859, Richard Owen, On the Classification and Geographical Distribution of the Mammalia, London: J. W. Parker and Son, page 29
  In the centre of the fleshy membrane is an aperture leading into a deep cavity, at the bottom of which is placed a prominent piston that may be retracted by muscular fibres provided for the purpose.
  1860, Samuel Hannaford, chapter 7, in Sea and River-side Rambles in Victoria, page 53
2. (optics) A hole which restricts the diameter of the lightpath through one plane in an optical system.
  1. (astronomy, photography) The diameter of such a hole which restricts the width of the lightpath through the whole system. For a telescope, this is the diameter of the objective lens.
3. (space flight, communication) The (typically) large-diameter antenna used for receiving and transmitting radio frequency energy containing the data used in communication satellites, especially in the geostationary belt. For a comsat, this is typically a large reflective dish antenna; sometimes called an array.
4. (mathematics, rare, of a right circular cone) The maximum angle between the two generatrices.