aperture

aperture is a   Noun

Definition:

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

Synonyms: