Joined so that one can reach the other. Connection holds two things together while leaving them two.

A single connection between two things, and the means of crossing it. One ring of a chain, useful only because it holds two others.

Established correspondences between two sets. A mapping carries nothing across but the relation itself.

Relation shortened to its attribute name — the link's declared kinship to what it points at.

Standing in a connection to something else. Relation is a fact about the pair, never about either alone.

An address by which a resource can be reached. It says where, not what.