Difference between an "IS" or "AS" function/procedure declaration PL/SQL

Question

When declaring a procedure/function in an ORACLE database, we can find this :

   CREATE FUNCTION get_bal(acc_no IN NUMBER) 
   RETURN NUMBER 
   IS acc_bal NUMBER(11,2);
   BEGIN 
   ...

In the example above, we use "IS" just before the variable declaration. But I've also seen that we can use "AS".

What's the difference between "IS" and "AS"? Thanks in advance.