Definition
Zulu is the aviation term for Coordinated Universal Time (UTC), the single worldwide time standard used in flight planning, weather products, NOTAMs, flight plans, and ATC communications. It is the same clock everywhere on Earth and does not change for time zones or daylight saving time. Times are written with a trailing 'Z' (for example, 1430Z) and expressed in a 24-hour format.
Plain English
Zulu is the one shared clock that aviation uses worldwide, so everyone -- pilots, controllers, and weather forecasters -- is talking about the same moment in time, no matter what time zone they are in.
Context Anchor
Seen in preflight weather briefings and aviation weather products as “AIRMET Zulu.”
Derivation
In the international phonetic alphabet, the letter 'Z' is spoken as 'Zulu.' Because UTC times are written with a 'Z' suffix (e.g., 1200Z), pilots simply say 'Zulu' for that 'Z.' The 'Z' itself comes from the naval/military time-zone system, where the zero-offset zone (Greenwich) was labeled 'Z.'
Why Pilots Care
Using one universal time reference prevents scheduling errors and keeps all aviation activity synchronized across different regions and time zones.
Intuition Check
Zulu does not mean Zulu time here. In this AIRMET context, Zulu is the label for icing and freezing-level information.
Example Sentence 1
The AIRMET was issued at 1500Z and is valid until 2100Z.
Example Sentence 2
The pilot filed the flight plan with a departure time of 0900 Zulu to match the controller’s clock.