In baseball, the "strike zone" is defined as the area over home plate that extends from the midpoint between a batter's shoulders and the top of their pants (typically referred to in informal terms as the area between the armpits and the knees). A pitch that crosses through this zone is typically considered a strike if the batter does not swing.
In summary, the area over home plate between the armpits and the knees is a crucial part of the strike zone, which is integral to the game’s rules regarding pitching and batting.