How is it defined?
Zero or Prime Meridian Time
"Zulu" time , more commonly know as "GMT" ( Greenwich Mean Time ) is time at the Zero Meridian. Our natural concept of time is linked to the rotation of the earth and we define the length of the day as the 24 hours it takes (on average) the earth to spin once on its axis.
As time pieces became more accurate and communication became global, there needed to be a point from which all other world times were based. Since Great Britain was the world's foremost maritime power when the concept of latitude and longitude came to be, the starting point for designating longitude was the Prime Meridian , which is zero degrees longitude and runs through the Royal Greenwich Observatory, in Greenwich, England .
When the concept of time zones was introduced , the "starting" point for calculating the different time zones was agreed to be the Royal Greenwich Observatory.
Unfortunately the Earth does not rotate at exactly a constant rate. Due to various scientific reasons and increased accuracy in measuring the earth's rotation, a new timescale, called Universal Time Coordinated or Coordinated Universal Time (UTC), has been adopted and replaced the term GMT.
The Navy, as well as civil aviation, uses the letter "Z" (phonetically "Zulu") to refer to the time at the Prime Meridian.
NOAA satellites use Zulu Time or Coordinated Universal Time (UTC) as their time reference. The satellite images that appear on NOAA's Web sites are stamped in Zulu time.
The Department of the Navy serves as the United States official timekeeper, with the Master Clock facility at the U.S. Naval Observatory, Washington, D.C.