In baseball, a strike is a mark against a batter during a plate appearance. A batter that receives three strikes during a plate appearance makes an out and is said to have struck out.
There are several ways to receive a strike:
Swinging at a pitched ball and failing to hit it. (swing and miss, strike swinging)
Failing to hit a pitched ball which is called a strike—determined to be in the strike zone—by the umpire. (called strike, strike looking)
Swinging at a pitched ball and hitting it into foul territory when there are fewer than two strikes in the at-bat. (foul ball)
Bunting at a pitched ball and hitting it into foul territory. This counts as a foul strike regardless of the number of strikes already charged to the batter.
Bunting at a pitched ball and failing to hit it. This counts as a swing and miss.
Touching a pitched ball while striking at it with the bat.
Touching a pitched ball while it is in the strike zone. (Intentional touching of a pitched ball is not allowed; see hit by pitch.)
Swinging at a pitched ball and foul tipping it into the catcher's glove. (foul tip)
A normal foul strike cannot count against the batter as his third strike; the third strike must be a swing and miss, called strike, touched ball or foul tip.
In baseball, the strike zone is a conceptual rectangular area over home plate which defines the boundaries through which a pitch must pass in order to count as a strike when the batter does not swing.
Unofficially, the strike zone in Major League Baseball is often enforced as being from the knee of the batter to no higher than his belt, although there are a handful of umpires known to call the 'high' strike.
While baseball rules provide a precise definition for the strike zone, in practice it is up to the judgment of the umpire to decide whether the pitch passed through the zone.