NOUN (3)

1. a device that controls amount of light admitted;

2. a natural opening in something;

3. an man-made opening; usually small;

The Collaborative International Dictionary of English v.0.48:

Aperture \Ap"er*ture\ (?; 135), n. [L. apertura, fr. aperire.
   See Aperient.]
   1. The act of opening. [Obs.]
      [1913 Webster]

   2. An opening; an open space; a gap, cleft, or chasm; a
      passage perforated; a hole; as, an aperture in a wall.
      [1913 Webster]

            An aperture between the mountains.    --Gilpin.
      [1913 Webster]

            The back aperture of the nostrils.    --Owen.
      [1913 Webster]

   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.
      [1913 Webster]

   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.
         [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

89 Moby Thesaurus words for "aperture":
   access, aisle, alley, ambulatory, arcade, artery, avenue, bore,
   breach, break, broaching, cavity, channel, chasm, check, chink,
   clearing, cleft, cloister, colonnade, communication, conduit,
   connection, corridor, covered way, crack, crevice, cut, defile,
   disclosure, discontinuity, exit, fenestra, ferry, fissure, fistula,
   fontanel, foramen, ford, gallery, gap, gape, gash, gat, gulf,
   hiatus, hole, hollow, inlet, interchange, intersection, interstice,
   interval, junction, lacuna, lane, laying open, leak, opening,
   opening up, orifice, outlet, overpass, pass, passage, passageway,
   perforation, pinhole, pore, portico, prick, puncture,
   railroad tunnel, rupture, slash, slit, slot, space, split, stoma,
   throwing open, traject, trajet, tunnel, uncorking, underpass,
   unstopping, vent, yawn