To put a shell around a computer to protect it from intruders.
To harden a system, the following techniques need to be done: The Operating System Software and the
exposed services should be patched
with the latest security fixes; the defaults should be removed; all unnecessary
services should be shut down; and packet-filtering software should be
Operating System Software; Patch.
Graham, R. Hacking Lexicon. [Online, 2001.] Robert Graham Website.