Limits defining behavior at edges.

Thin layer where fluid meets surface.

Constraints defining values at edges.

Interface separating different crystal orientations.

Demarcation separating two distinct areas.

Limits that define separate areas.

Thin layers where properties change.

Limits defining range of function.

Border separating two sovereign nations.

Panel determining borders between territories.

In thermal equilibrium, each phase (i.e. liquid, solid etc.) of physical matter comes to an end at a transitional point, or spatial interface, called a phase boundary, due to the immiscibility of the matter with the matter on the other side of the boundary.