the schedule of operations just before the firing of a rocket, the detonation of an explosive device, etc.; also, the counting off, in reverse order, of units of time in such a schedule
See countdown in American Heritage Dictionary 4
(kountˈdounˌ)
noun
The counting backward aloud from an arbitrary starting number to indicate the time remaining before an event or operation, such as the launching of a missile or space vehicle.
The checks and preparations carried out during this activity.