Countdown

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Countdown

1. A step-by-step process that culminates in a climactic event, each step being performed in accordance with a schedule marked by a count in inverse numerical order; specifically, this process is used in leading up to the launch of a large or complicated rocket vehicle, or in leading up to a captive test, a readiness firing, a mock firing, or other firing test.
2. The act of counting inversely during this process.
In sense 2, the countdown ends with T-time; thus, T minus 60 minutes indicates there are 60 minutes to go, excepting for holds and recycling. The countdown may by hours, minutes, or seconds. At the end, it narrows down to seconds, 4-3-2-1-0. See plus count.

References

This article is based on NASA's Dictionary of Technical Terms for Aerospace Use