A Starfleet general order instructing a starship to destroy all life on a planet's surface, which Kirk issues over Eminiar VII as a bluff backed by real force.
General Order 24 directs a starship to lay waste to the whole inhabited surface of a planet. It is the most extreme command a starship captain is shown giving in the original series, and it can be given with a time limit so that it takes effect unless the captain cancels it.
The order appears in A Taste of Armageddon. Held prisoner on Eminiar VII, whose computer-run war with Vendikar has declared the Enterprise crew casualties, Kirk contacts Scott and gives General Order 24 with a deadline of two hours. Scott readies the ship to carry it out.
The threat, together with Kirk's destruction of the war computers, forces the Eminian council to face the prospect of real warfare and to open talks with Vendikar. Kirk then cancels the order before the deadline runs out.