These words all denote an empty space or a hole in something. Opening , the most general, refers to any vacant or unobstructed space, as hole or passage. It can be substituted for each of the other words here considered in many contexts: an opening cut in a fence for passers-by to watch the construction going on in an excavation; an opening in the ground serving as a sump. An opening in a forest is a tract where few or no trees grow. Thus an opening may be either natural or contrived. Gap usually applies to a wide crack and suggests a deviation from the normal or regular conformation: a gap between two teeth; a large gap in the wall of the bombed building. Gap is also used abstractly, indicating a break in continuity: a gap in one’s memory. It is also a narrow, steep space or defile between two cliffs or hills: The ship passed through the gap between the heads; cattle driven through the gap into the grassy valley.

Aperture is a more formal word than opening , and applies to any opening or cleft in a surface, regardless of the nature of the cause: windows regarded as apertures in walls; apertures in the leaves of a diseased tree. The aperture of a camera is an opening , often adjustable in diameter, through which light enters the lens. Orifice , which is derived from the Latin word for mouth, refers to an aperture or opening into a cavity or enclosed place, and therefore suggests a point of access or entry: the orifice of a chimney; the orifice of a test tube; Smoke belched from the orifice of the volcano. Orifice is much used in medical contexts in reference to openings into body cavities. The pulmonary orifice , for example, is the opening of the pulmonary artery into the heart. Interstice is a very formal word suitable only in contexts where preciseness of description is necessary. It refers to one of a series of narrow spaces or openings between adjoining things or parts.

• The interstices of a sweater knitted with thick wool are comparatively large; If the interstices in a fisherman’s net are too large, too many fish will escape through the meshes.