2. 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.
3. (Opt.) The diameter of the exposed part of the object
glass of a telescope or other optical instrument; as, a
telescope of four-inch 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[deg] aperture.