countdown

n. 倒计数
【电】 指示串联中的操作数固定的记录减少

count.down

n.（名词）
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.
倒数报时：从任意的一个数字开始倒着数，以表明距离某件事或操作（例如发射导弹或宇宙飞船）的时间
The checks and preparations carried out during this activity.
准备过程，准备工作：在倒数报时期间所进行的检查和准备