Aperture

noun

1. An opening, gap, or hole, usually small and narrow

   "an aperture in a wall"

2. Something which restricts the diameter of the light path through one plane in an optical system.
3. The diameter of the aperture (in the sense above) which restricts the width of the light path through the whole system. For a telescope, this is the diameter of the objective lens.

   "This telescope has a 100cm aperture."

4. (communications) 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. (of a right circular cone) The maximum angle between the two generatrices.

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