Full Title or Meme
An imaginary definable perimeter encompassing all the critical functions in an INFOSEC product and separating them from all other functions within the product.
All security boundaries have gateways that allow access to the protected Resources. Determination of what accesses are permitted across a boundary is never more than an approximation of an impenetrable boundary.
- NIST Glossary https://csrc.nist.gov/Glossary/?term=4819