Hi,
maybe you would be more successful if you knew what you use. In sqlplus, it is called substitution variable, and it is described in SQL*Plus User's Guide and Reference book. It is available with other Oracle documentation books e.g. online on
http://tahiti.oracle.com/
For undefining it, you should use UNDEFINE sqlplus command. For 11gR2 (it would be nice if you posted your Oracle version), it is described here:
http://docs.oracle.com/cd/E11882_01/...9.htm#i2699781
(maybe you are using other client than sqlplus; in that case, consider consulting its online help about substitution variables; it should be accessible from menu or by F1 button)