State the number of definitions of firewall

Though there exist a number of definitions of firewall, in simplest terms it can be defined as "a mechanism used to protect a trusted network from an entrusted network". It can also be defined as "A firewall is a system, or group of systems that enforce an access control policy between two networks, and thus should be viewed as an implementation of policy". From the above two definitions, it can be understood that a firewall is only as good as the security policy it supports.