security key (n)~ An identifier used by two APPC logical units (LUs) to validate security when a session is activated. The security key performs a function similar to that of a password but at the LU-LU session level rather than at the TP-conversation level. security label (n)~ A secure e-mail feature that lets you add sensitivity labels such as Internal Use Only to the message header. Security labels in your organization are controlled by security policies set up by your e-mail administrator.