Source : Webster's Revised Unabridged Dictionary (1913)

Aperture \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.

Source : WordNet®

aperture
     n 1: a device that controls amount of light admitted
     2: a natural opening in something
     3: an man-made opening; usually small