A made way across a gap that would otherwise stop passage. A bridge belongs to neither side and serves both.

The opening a door stands in — the threshold itself: never locked by the leading-in, per the house's own law.

A controlled way through into somewhere else. A gateway is a boundary built for passage.

A correspondence made explicit — each thing of one kind pointed at its counterpart in another. A mapping is a noun in its own right: the finished bridge, not the act of building it.

The reach from one end to another. Also a marked stretch within a line of text.