A changeup is a type of pitch in baseball that looks like it's coming fast but arrives much slower than expected. The pitcher grips and throws the ball in a special way that makes their arm motion look exactly like a fastball, but the ball comes out of their hand with less speed. When a batter swings expecting a 90-mile-per-hour fastball and gets a 75-mile-per-hour changeup instead, they usually swing too early and miss.

The changeup works through deception. The whole point is to fool the batter into thinking one thing is coming when something else arrives. A good changeup pitcher can make batters look silly, swinging at air while the slow ball floats past them into the catcher's mitt.

Outside baseball, people use changeup to describe any unexpected variation in pattern or pace. If a teacher usually gives multiple-choice tests but suddenly assigns an essay, a student might say, “That was a real changeup.” When someone keeps doing the same thing and then suddenly switches tactics, they're throwing you a changeup. The word captures that moment when you were ready for one thing but got something different.