A countdown is the act of counting backward from a number to zero, usually to mark the time remaining until something important happens. When a rocket prepares to launch, mission control announces “10, 9, 8, 7...” until they reach zero and the engines ignite. On New Year's Eve, crowds shout the final ten seconds before midnight, counting down to the start of a new year.

Countdowns build excitement and help everyone focus on the same moment. They give people time to prepare: a teacher might count down from five to signal that cleanup time is ending, or a photographer might count “3, 2, 1” before snapping a picture. The word can also describe the period of time itself, like when you're in the final countdown before a big performance or when there's a countdown of three days until your birthday.

The practice became famous during the Space Age in the 1960s, when NASA engineers used countdowns to synchronize the hundreds of systems that had to work perfectly for a rocket launch. Today we use countdowns for everything from microwave timers to video game matches, turning the simple act of counting backward into a way to make any moment feel more dramatic and purposeful.