[countdown] {n.}. {Space English}, {informal} 1. A step-by-step process which leads to the launching of a rocket.

Countdown starts at 23:00 hours tomorrow night and continues for 24 hours.

2. Process of counting inversely during the acts leading to a launch; liftoff occurs at zero.

3. The time immediately preceding an important undertaking, borrowed from Space English.

We're leaving for Hawaii tomorrow afternoon; this is countdown time for us.