A Starfleet regulation forbidding uncoded messages on an open channel when transmissions may be monitored in battle, which Kirk and Spock use to deceive Khan.
Regulation 46A states that if transmissions are being monitored during battle, no uncoded messages are to be sent on an open channel. It is quoted by Saavik in Star Trek II: The Wrath of Khan.
While Kirk is on the Regula I station and Khan is listening in from the Reliant, Spock reports that repairs to the Enterprise will take days and that the ship cannot rescue the landing party in time. Saavik, hearing this, is troubled that the captain appears to have broken the regulation by speaking so openly.
Kirk later explains that the two of them were following it all along. Spock's report was a simple code in which days stood for hours, so the Enterprise is ready far sooner than Khan believes and is able to beam the party up from the Genesis cave.