A personal code spoken by a senior Starfleet officer to confirm identity before the computer will accept restricted orders such as self-destruct or a transfer of command.
A command authorisation code is the personal password of a senior officer. The computer checks both the voice and the code before it will carry out orders too serious to accept from anyone, such as arming the self-destruct system, moving command functions or opening sealed files.
In Star Trek III: The Search for Spock, Kirk, Scott and Chekov each speak a code in turn to set the Enterprise to destroy itself under the feet of a Klingon boarding party. Picard uses his own code in Star Trek: First Contact, when he, Crusher and Worf arm the Enterprise-E's destruct sequence.
The system can be turned against a crew. In Brothers, Data, acting under a recall signal from his creator, copies Picard's voice, moves control of the ship to himself and seals it behind a code so long that nobody else could hope to repeat it.