The Rhine is the boundary between France and Germany.
the city boundary
the river marks the boundary between the two regions
Some of the boundary between Germany and Poland follows the course of the river Oder. The Ural mountains mark the boundary between Europe and Asia.
personal boundary = your limit, your comfort zone
the boundary of a physical area is the edge, where one area stops and another starts. A boundary is also the point at which one thing starts to become another
It is beyond the boundary of human knowledge.
The mission took the direct route by air to the boundary.