在棒球比赛中，打击者经裁判判定获得三个好球后，即被三振。记一出局。 另外，三振为投手之统计数据。打者被判定三振出局之同时，当时投球之投手即可获计一次夺三振。三振能力被认为是投手能力的重要指标之一。

棒球规则中，规定两好球之后击球界外球时不计好球数，因此打者可继续攻击，但如擦棒界外球（球往后飞行未超过头部高度的界外球）被捕手接捕，则会被判好球三振一次，但若是界外高飞球被接捕，则计界外置杀而非三振。

此外，如第三个好球数投出后，捕手未能接捕投手投出的球，在一垒没有跑者时或虽有跑者但已有二人出局，则打者可试图往一垒跑，若能在捕手传球至一垒前安全上垒，则虽然记录上仍计一次三振，但打者上垒且不计一次出局，这种情形称作不死三振。

In baseball or softball, a strikeout (or strike-out) occurs when a batter accumulates three strikes during a time at bat. It usually means the batter is out. A strikeout is a statistic recorded for both pitchers and batters, and is denoted by K.

Although a strikeout suggests that the pitcher dominated the batter, the free-swinging style that generates home runs also leaves batters susceptible to striking out. Some of the greatest home run hitters of all time — such as Alex Rodriguez, Gorman Thomas, Reggie Jackson, and Sammy Sosa — were notorious for striking out.