A countdown is a sequence of counting backward to indicate the seconds, days, or other time units remaining before an event occurs or a deadline expires. Typical events for which a countdown is used include the launch of a rocket

Rocket

A rocket is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine. In all rockets, the exhaust is formed entirely from propellants carried within the rocket before use. Rocket engines work by action and reaction...

 or spacecraft

Spacecraft

A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....

, the detonation of a bomb, the start of a race, and the start of the New Year

New Year

The New Year is the day that marks the time of the beginning of a new calendar year, and is the day on which the year count of the specific calendar used is incremented. For many cultures, the event is celebrated in some manner....

. In an early attestation of a countdown, it started a rowing race at Cambridge University. The first known association with rockets was in the 1929 German science fiction movie Die Frau im Mond by Fritz Lang

Fritz Lang

Friedrich Christian Anton "Fritz" Lang was an Austrian-American filmmaker, screenwriter, and occasional film producer and actor. One of the best known émigrés from Germany's school of Expressionism, he was dubbed the "Master of Darkness" by the British Film Institute...

 to increase the drama of the launch sequence of the story's lunar rocket.
A countdown is a carefully devised set of procedures ending with the ignition of a rocket's engine.