counting backward from an arbitrary number to indicate the time remaining before some event (such as launching a space vehicle)
"The control tower initiated a ten-second countdown before firing the rockets to launch the space shuttle."
A count backward in fixed units to the time of some event, especially the launch of a space vehicle.
"The mission control team initiated a dramatic countdown before the rocket's powerful engines ignited at liftoff."
In plain English: A countdown is when you count backward from a big number to zero, usually because something exciting is about to start or happen soon.
"The countdown to New Year's Eve began at midnight on Christmas Day."
Usage: Use this noun specifically when referring to a sequence counting backward toward an upcoming event or deadline. It is distinct from simply stating how much time remains until something happens.
To count down.
"The rocket launch was scheduled to begin when the countdown reached zero."
This word is formed from the combination of "count" and "down." It originally meant to count backwards, typically toward a specific event or time.