Aperture

aperture 1. 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. The back aperture of the nostrils. 3. (Optics) Something which restricts the diameter of the light path through one plane in an optical system. 4. (Astronomy and photography) The diameter of the aperture (in the sense above) which restricts the width of the ligth 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. Note: 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.

Miss Information says: An oppening or hole in something for viewing. Example: My fence has an aperture at eye level. An aperture should not to be confused with a lens. Aperture does not refer to viewing.

A simplified definition of aperture would be: An opening or hole. Also refers to that part of a camera which controls how much light may enter.