For other meanings of the term countdown, see Countdown (disambiguation).

A countdown is the backward counting to indicate the seconds, days, etc. remaining before an event. Typical events for which a countdown is used include the launch of a space rocket, the explosion of a bomb, the start of a race or New Year.

In the context of a rocket launch, the "T minus Time" is the time before launch, e.g. "The countdown was held at T minus 5 minutes for 3 minutes and 40 seconds". The last ten seconds are usually counted down aloud "10, 9, 8, 7, 6, 5, 4, 3, 2, 1, Lift off!"

The first known use with respect to launching a rocket occurred in Fritz Lang's 1929 film Frau im Mond. In the film, after each number the words "seconds to go" was repeated.

The countdown is a inverse counting from a certain bigger number to a smaller one. Often this countdown starts at a bigger number until zero, but sometimes the countdown stops before reaching zero.