\Ap"er*ture\ (?; 135), n. [L. apertura, fr. aperire.
See {Aperient}.]
1. The act of opening. [Obs.]
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.