What qualifies as an illegal substitute?

An illegal substitute is defined as a player who enters the game in a manner that violates the established rules of baseball regarding substitutions. When a player enters in the wrong position in the lineup, it disrupts the order in which players are supposed to bat and can lead to significant confusion during the game. This situation is a clear violation of the rules and makes the substitute illegal because it affects the integrity of the game and can have implications for the strategy being used by both teams.

In contrast, while some other scenarios concerning players can be problematic, they don't specifically classify an entry as an illegal substitute under the rules. A player not registered with the team pertains to eligibility rather than substitution legality, and the same goes for a player who was previously ejected—this is more about participation eligibility. Likewise, changing a uniform number does not inherently make a substitution illegal as long as the player is properly registered and in compliance with the rules at the time of substitution. Thus, the entry in the wrong position in the lineup is what directly constitutes an illegal substitute.