Tuberculin: A protein extracted from Mycobacterium tuberculosis that is used in a skin test to determine if a person has been exposed to tuberculosis. The tuberculin preparation in most common use today is purified protein derivative (PPD) tuberculin. PPD is injected (or multiple-punctured) into the skin. If a reaction is seen at and around the site of injection, the test is positive. The PPD tuberculin test done by intradermal injection is also known as the Mantoux test.