aperture

noun

1. 1

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

   “an aperture in a wall”

2. 2

   Something which restricts the diameter of the light path through one plane in an optical system.

3. 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. 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. 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.”