Dictionary.net

1. The code you put in a system (say, a telephone switch) tomake sure that the users can't do any damage. Since usersalways want to be able to do everything but never want tosuffer for any mistakes, the construction of a firewall is aquestion not only of defensive coding but also of interfacepresentation, so that users don't even get curious about thosecorners 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 tofix the bug, and once to insert a firewall which would havearrested the bug before it did quite as much damage.