A strikeout occurs when a pitcher throws any combination of three swinging or looking strikes to a hitter. (A foul ball counts as a strike, but it cannot be the third and final strike of the at-bat. A foul tip, which is caught by the catcher, is considered a third strike.)
The batter is automatically out on a strikeout, unless the catcher does not cleanly hold onto the baseball or if the baseball hits the dirt. If the catcher does not catch the third strike, the batter may attempt to run to first base -- if it is open or if there are two outs. However, even if the batter reaches first base safely, the pitcher and the batter are still credited with a strikeout in the scorebook.
In the scorebook, a strikeout is denoted by the letter K. A third-strike call on which the batter doesn't swing is denoted with a backward K.
Before 1858, the strikeout required three pitches be offered at and missed. However, in 1858, the addition of the called strike was implemented and the strikeout rule has changed very little since.
In A Call
"whiff," "K," "punch out" (for pitchers); as a verb in place of "strikes out": "fans," "rings up," "whiffs," "K's," "punches out"