Here is the example:
SQL> set define off;
SQL> select * from dual where dummy='&var';
no rows selected
SQL> set define on
SQL> /
Enter value for var: X
old 1: select * from dual where dummy='&var'
new 1: select * from dual where dummy='X'
D
-
X
With set define off, it took a row with &var value, prompted a user to enter a value for it and replaced &var with the entered value (in this case, X).