An illegal pitch may be quick pitch (i.e. a pitch made before the batter is properly set in the batter's box), a pitch made while the pitcher is not in contact with the pitching rubber, or one in which he takes an extra step while making his delivery.
Illegal pitch can also refer to instances in which the pitcher has been doctoring the baseball or to a spitball. In this case, it is not the pitch that is illegal, but the foreign substance applied to the baseball or marks made on it. The penalties in such cases can be more severe.