An Aperture

1. An opening, gap, or hole, usually small and narrow
2. (optics) Something which restricts the diameter of the light path through one plane in an optical system.
3. (astronomy, photography) 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.
4. (spaceflight, communication) 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. (mathematics, rare, of a right circularcone) The maximum angle between the two generatrices.

Usage notesEdit

The aperture of microscopes is often expressed in degrees, called also the angular aperture, which signifies the angular breadth of the pencil of light which the instrument transmits from the object or point viewed; as, a microscope of 100° aperture.