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. [Jargon File]