Hi akhil
The DEFINE command is used to load a CHAR variable with data.This can be used only with character data.If numeric data is loaded be sure to use appropriate conversion if it will be used for comparison purposes.
The EXAMPLE is

SQL>DEFINE POS=MANAGER

The above example assigns the text value "MANAGER" to the variable POS.If &POS is used in a script that follows,the value "MANAGER" will be substituted at runtime.