Hardening is the process of optimising a system's security configuration. It is a term usually applied to operating systems. Hardening NT/2000 would, for example, require you to
* Disable unused and Guest accounts
* Disable unused NT Services
* Shut down all un-neccesary services
* Disallow all unnecessary applications
It is a process that should be applied to all operating systems.
Many firewalls run on pre-hardened operating systems. Microsoft's ISA server, for example, includes a built-in System Hardening Wizard that “locks down” the underlying Windows 2000 operating system by disabling services unrelated to ISA Server.