Aperture

Part of speech: noun (n.)

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.