What constitutes an "illegal pitch"?

An illegal pitch is defined by specific actions or failures to comply with the established rules of baseball. One primary way a pitch can be deemed illegal is if it is delivered while the pitcher is not in contact with the pitcher's rubber. The pitcher is required to have at least one foot in contact with the rubber prior to making the pitch. This regulation ensures that the pitcher has a proper starting position, maintaining a balance between offensive and defensive play.

While the other options present different scenarios that might occur during a game, they do not align with the rules defining an illegal pitch. For instance, the speed of the pitch is not a factor in rendering it illegal, nor is the pitch's failure to reach home plate; these situations can be a part of regular gameplay but do not violate any rules concerning the legality of the pitch itself. Similarly, using a foreign substance on the ball pertains to different regulations regarding pitch integrity but is not classified as an illegal pitch in the same direct manner as the requirement for the pitcher to be in contact with the rubber.