From Halopedia, the Halo wiki
LUX is a United Nations Space Command flagword to be declared prior to a tactical nuclear deployment.
As part of the UNSC's ongoing URNA Southwest Campaign during the Battle for Earth in November, 2552, LUX was declared in anticipation of a tactical nuclear deployment against multiple shielded Covenant positions centered around Free-fire Area OZONE in southeastern Arizona. The UEG issued an emergency evacuation broadcast as part of the order.
"Lux" is the Latin word for "light."