1. The code you put in a system (say, a telephone switch) to make
sure that the users can't do any damage. Since users always want to
be able to do everything but never want to suffer for any mistakes,
the construction of a firewall is a question not only of defensive
coding but also of interface presentation, so that users don't even
get curious about those corners of a system where they can burn
themselves.
2. Any sanity check inserted to catch a {can't happen} error. Wise
programmers often change code to fix a bug twice: once to fix the
bug, and once to insert a firewall which would have arrested the bug
before it did quite as much damage.
[glossary]
[Reference(s) to this entry by made by: {can't happen}]