an empty area (usually bounded in some way between things)

the outermost level of the land or sea

have an opening or passage or outlet

an opening at the corner or on the side of a billiard table into which billiard balls are struck

a gap or vacant space between two teeth

small gaps in the myelin sheath of medullated axons

a natural opening or perforation through a bone or a membranous structure

an opening (especially a gap in a dike or fortification)

the outermost level of the land or sea