Real Dictionary

countdown
singular noun: A countdown is the counting aloud of numbers in reverse order before something happens, especially before a spacecraft is launched.

countdown in American English
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
noun: the backward counting in fixed time units from the initiation of a project, as a rocket launching, with the moment of firing designated as zero

countdown in British English
noun: the act of counting backwards to time a critical operation exactly, such as the launching of a rocket or the detonation of explosives
verb: to count numbers backwards towards zero, esp in timing such a critical operation