aperture

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 ]

☞ 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. [ 1913 Webster ]