Omega three
From Halopedia, the Halo wiki
Code Omega Three was a UNSC emergency code. The code, to be used during combat operations, chilled any soldier to the core.
Omega Three was the panic code. An order to break and run no matter the cost. This code was used only in extreme situations, when a combat sisutation suddenly turned extremely grim for the UNSC forces and they would be massacred if they did not immediately retreat.
