count·down (kount?doun?)n.1. 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.2. The checks and preparations carried out during this activity.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.countdown (?ka?nt?da?n) nthe act of counting backwards to time a critical operation exactly, such as the launching of a rocket or the detonation of explosivesvb (intr, adverb) (Mathematics) to count numbers backwards towards zero, esp in timing such a critical operationCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014count?down (?ka?nt?da?n) n. 1. the backward counting from the initiation of a project, as a rocket launching, with the moment of firing designated as zero. 2. the final preparations made during this period. 3. any period of increased activity before a deadline. [1950?55] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.countdownThe step-by-step process leading to initiation of missile testing, launching, and firing. It is performed in accordance with a pre-designated time schedule.Dictionary of Military and Associated Terms. US Department of Defense 2005.