What is the difference between "AS" and "IS" in an Oracle stored procedure?

I see Oracle procedures sometimes written with "AS", and sometimes with "IS" keyword.

CREATE OR REPLACE Procedure TESTUSER.KILLINSTANCE (INSTANCEID integer) **AS**
...

vs.

CREATE OR REPLACE Procedure TESTUSER.KILLINSTANCE (INSTANCEID integer) **IS**
...

Is there any difference between the two?

None whatsover. They are synonyms supplied to make your code more readable:

FUNCTION f IS ...

CREATE VIEW v AS SELECT ...

One minor difference...

They are synonyms for packages and procedures, but not for cursors:

This works...

cursor test_cursor
is
select * from emp;

... but this doesn't:

cursor test_cursor
as
select * from emp;

"IS" and "AS" act as a synonym while creating procedures and packages but not for a cursor, table or view.

Here's another difference (in 10g, at any rate)

Given a loose object type:

CREATE TYPE someRecordType AS OBJECT
(
   SomeCol VARCHAR2(12 BYTE)
);

You can create a loose Table type of this object type with either AS or IS

CREATE OR REPLACE TYPE someTableType
        IS {or AS} TABLE OF someRecordType;

However, if you create this same table type within a package, you must use IS:

CREATE OR REPLACE PACKAGE SomePackage IS
    TYPE packageTableType IS TABLE OF someRecordType;
END SomePackage;

Use of AS in the package yields the following error:

Error(2,30): PLS-00103: Encountered the symbol "TABLE" when expecting one of the following: object opaque

According to TutorialsPoint

The AS keyword is used instead of the IS keyword for creating a standalone procedure.

and considering previous answers,

I guess

AS is for stand alone (outside of any block, subprogram, package) entities

and

IS is for embedded (within a block, subprogram or package) entities.

.