Stands for Unauthorized Absence Light. Unauthorized absence, is punishable under article 86 of the Uniform Code of Military Justice (UCMJ). This refers to a traffic light which causes military servicemen and women to arrive late to work (or formation) due to the longer than normal time for the light to turn green.
Sergeant: Why were you late this morning!?
PFC: Uh, I was stuck in traffic Sarn't...
Sergeant: I guess you got caught at the UA Light huh? Good, stand-the-fuck-by.