aperture

noun5

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

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

   • 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.