In baseball or softball, a strikeout (or strike-out) occurs when a batter accumulates three strikes during a time at bat. It means the batter is out, unless the third strike is not caught by the catcher and the batter reaches first base safely as a result. A strikeout is a statistic recorded for both pitchers and batters, and is usually denoted by the letter K, or sometimes by the initialism SO.

Source: Wikipedia