Etymology
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countdown (n.)

also count-down, "the counting down of numerals in reverse order to zero before a significant event," also the preparations made during this time, 1953, American English, in early use especially of launches of rockets or missiles, from the verbal phrase (attested by 1954); see count (v.) + down (adv.).

updated on April 24, 2018

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Definitions of countdown from WordNet

countdown (n.)
counting backward from an arbitrary number to indicate the time remaining before some event (such as launching a space vehicle);
Etymologies are not definitions. From wordnet.princeton.edu, not affiliated with etymonline.