What is the strike zone?

The official strike zone is the area over home plate from the midpoint between the batter's shoulders and the top of his uniform pants (measured when he is in his stance and ready to swing) down to a point just below the kneecap. For a strike call, part of the ball must cross over part of home plate within that area.

The home-plate umpire calls balls and strikes after every pitch has passed the batter. If the batter makes contact with the pitch, it is automatically a strike. The current vertical limits have been in place since 1996.

Source: MLB, Strike Zone (mlb.com/glossary/rules/strike-zone).