aperture

plural apertures

1. A small or narrow opening, gap, slit, or hole.

   an aperture in a wall

   3 more examples

   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

   The door was opened — ‘on the chain.’ The old lady peered at us through an aperture of about six inches.

   1897, Richard Marsh, The Beetle

   Synonyms: chink

2. engineeringA hole which restricts the diameter of the lightpath through one plane in an optical system.
   1. artsThe 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.

      This telescope has a 100 cm aperture.

3. space flightcommunicationThe (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. raremathematicsThe maximum angle between the two generatrices.

   If the generatrix makes an angle θ to the axis, then the aperture is 2θ.