define usage:
define定义的是用户变量。
define X=32
pass a char type whose value is 32 to the variable X.
Use &X to implement in pl/sql block;
define x
display the value of user variable x.
eg. DEFINE X = "32" (CHAR)
define
display the values of all user variable
undefine（undef)
del the user variable defined.
Here is the example:
10:54:50 lab@ORCL>define x=345
10:54:56 lab@ORCL>define x
DEFINE X = "345" (CHAR)
10:54:59 lab@ORCL>define
DEFINE _DATE = "2010-04-09" (CHAR)
DEFINE _CONNECT_IDENTIFIER = "orcl" (CHAR)
DEFINE _USER = "LAB" (CHAR)
DEFINE _PRIVILEGE = "" (CHAR)
DEFINE _SQLPLUS_RELEASE = "1002000100" (CHAR)
DEFINE _EDITOR = "Notepad" (CHAR)
DEFINE _O_VERSION = "Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
With the Partitioning, OLAP and Data Mining options" (CHAR)
DEFINE _O_RELEASE = "1002000100" (CHAR)
DEFINE GNAME = "lab@ORCL" (CHAR)
DEFINE _RC = "0" (CHAR)
DEFINE X = "345" (CHAR)
10:55:02 lab@ORCL>undefine x
10:55:10 lab@ORCL>define x
SP2-0135: 符号 x 未定义
10:55:15 lab@ORCL>
<
h2>Variable Usage:
定义一个绑定变量。
variable x number;
a type must follow;
this type should have precision, like varchar2(30), number(10), but varchar2;
use 
in the pl/sql block.
show a variable
print variable_name; (print x)
Let see an example
10:58:56 lab@ORCL>vaiable x varchar2(30)
SP2-0734: 未知的命令开头 "vaiable x ..." - 忽略了剩余的行。
10:59:11 lab@ORCL>variable x varchar2(30)
10:59:19 lab@ORCL>exec := 'sslfsjdlfjsl';
PL/SQL 过程已成功完成。
已用时间: 00: 00: 00.01
10:59:34 lab@ORCL>print x
X
--------------------------------
sslfsjdlfjsl
10:59:38 lab@ORCL>
Conclusion
define is give predefined variable; variable works as literally.