A division between two regions. Physicists frequently analyze only a part of a larger system, so that they do not need to keep track of everything. This usually simplifies the analysis, but requires an understanding of what happens at the boundary. Interesting computer simulations usually require a boundary.
Come, you lost Atoms, to your Centre draw,
And be the Eternal Mirror that you saw:
Rays that have wander'd into Darkness wide,
Return and back into your Sun subside.
From Farid al-Din Attar's
The Conference of the Birds