aperture (plural apertures)
- An opening; an open space; a gap, cleft, or chasm; a passage perforated; a hole; as, an aperture in a wall.
- An aperture between the mountains. --Gilpin.
- The back aperture of the nostrils. --Owen.
- (optics) Something which restricts the diameter of the light path through one plane in an optical system.
- (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. e.g. a telescope may have a 100 cm aperture.
- (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.
- (mathematics, rare, 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.
Usage notes 
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.
Related terms 
- aperture in Webster’s Revised Unabridged Dictionary, G. & C. Merriam, 1913
- aperture in The Century Dictionary, The Century Co., New York, 1911
- Plural form of apertura
- vocative masculine singular of apertūrus