A spitball is an illegal baseball pitch in which the ball has been altered by the application of a foreign substance such as saliva or petroleum jelly. This technique alters the wind resistance and weight on one side of the ball, causing it to move in an atypical manner. It may also cause the ball to "slip" out of the pitcher's fingers without the usual spin that accompanies a pitch. In this sense, a spitball can be thought of as a fastball with knuckleball action. Alternative names for the spitball are spitter, mud ball, shine ball, supersinker, or vaseline ball. A spitball technically differs from an emery ball, in which the surface of the ball is cut or abraded. Saliva or Vaseline smooths the baseball, while the emery paper roughens it. The general term for altering the ball in any way is doctoring.

In baseball, the pitch is the act of throwing the baseball toward home plate to start a play. The term comes from the Knickerbocker Rules. Originally, the ball had to be thrown underhand, much like "pitching in horseshoes". Overhand pitching was not allowed in baseball until 1884.