A countdown is a method of ensuring that a complex sequence of events leading up to a single goal is achieved properly. Examples include the detonation of an atomic bomb; however, only in testing, really, when the event is rigidly controlled. Other familiar examples include space launches, such as the Space Shuttle Terminal Count phase; these have been used since the dawn of rocket launches to ensure all systems are operating and personnel are safe.

Typically, a countdown is expressed in negative numbers from the desired event (launch, detonation, what-have-you). This is usually referred to as Time T, so events in the countdown occur at T minus (time). For space launches, the end of the countdown (liftoff) signals the beginning of the mission clock, which is expressed as T plus time.