In thermodynamics, the boundary separates the system from its surroundings.
Boundary - A specific point in a specific sequence of stratified rock that serves to separate time units, for example, the Pennsylvanian-Permian boundary.
Areas conjured by Zedd where a section of the Underworld also exists in the world of life simultaneously. The Boundaries are invisible unless contacted, when they give off a green glow. One Boundary separated Westland from The Midlands and one separated The Midlands from D'Hara. Another boundary separated the land of Bandakar from the rest of the world. To enter any of the boundaries meant certain death.