For the ENT episode of the same name, please see "Countdown".
For additional meanings of "Countdown", please see Countdown.
A countdown was a sequence of counting by number or interval of time to indicate the start or end of something.
As the USS Enterprise attempted to perform a light-speed breakaway factor in 1969, Spock gave a countdown from ten to one to indicate when the ship had approached the breakaway point. (TOS: "Tomorrow is Yesterday")
In 2254, Spock gave a countdown from ten to one for the firing of the laser cannon against the entry to the underground Talosian community. (TOS: "The Cage")
In 2266, Hikaru Sulu gave an intermittent countdown from ten to zero minutes for the promised destruction of the Enterprise by Balok of the First Federation. (TOS: "The Corbomite Maneuver")
As the USS Enterprise-D attempted to perform saucer separation in 2364, Data gave a countdown from six to one to indicate when to separate the ship. (TNG: "Encounter at Farpoint")
In 2368, Data gave a countdown from three to one for the lifting of a beam, so as to free Timothy. (TNG: "Hero Worship")