aperture

noun

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

   “an aperture in a wall”

   “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.’”

   “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.”

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

2. A hole which restricts the diameter of the lightpath through one plane in an optical system.
3. A hole which restricts the diameter of the lightpath through one plane in an optical system.

   “This telescope has a 100 cm aperture.”

4. 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.
5. (rare)The maximum angle between the two generatrices.

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