In baseball, a leadoff hitter is a batter who bats first in the lineup. It can also refer to any batter who bats first in any inning.

In baseball, the batting order or batting lineup is the sequence in which the members of the offense take their turns in batting against the pitcher. The batting order is the main component of a team's offensive strategy. In Major League Baseball, the batting order is set by the manager, who before the game begins must present the home plate umpire with two copies of his team's lineup card, a card on which a team's starting batting order is recorded. The home plate umpire keeps one copy of the lineup card of each team, and gives the second copy to the opposing manager. Once the home plate umpire gives the lineup cards to the opposing managers, the batting lineup is final and a manager can make changes only under the Official Baseball Rules governing substitutions. If a team bats out of order, it is a violation of baseball's rules and subject to penalty.