SQL>
SQL> def x
DEFINE X = "7500" (CHAR)
SQL> def
DEFINE _DATE = "" (CHAR)
DEFINE _CONNECT_IDENTIFIER = "XE" (CHAR)
DEFINE _USER = "RNTSOFT" (CHAR)
DEFINE _PRIVILEGE = "" (CHAR)
DEFINE _SQLPLUS_RELEASE = "1002000100" (CHAR)
DEFINE _EDITOR = "Notepad" (CHAR)
DEFINE _O_VERSION = "Oracle Database 10g Express Edition Release 10.2.0.1.0 - Production" (CHAR)
DEFINE _O_RELEASE = "1002000100" (CHAR)
DEFINE _RC = "0" (CHAR)
DEFINE X = "7500" (CHAR)
SQL> undefine x
SQL>