granica wewnętrzna in English:

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.