Bravo Zulu is a naval signal, typically conveyed by flaghoist or voice radio, meaning "Well Done"; in addition to U.S. naval forces, it has also been used as part of vernacular slang within NATO and other Allied naval forces. It can be combined with the "negative" signal, spoken or written NEGAT, to say "NEGAT Bravo Zulu", or "not well done".
According to the "Navy Data" reference site, operated by the U.S. Navy: "The term originates from the Allied Tactical Publication 1 (ATP 1), an Allied maritime tactical signals publication, which in the aggregate is For Official Use Only. Signals are sent as letters and/or numbers, which have meanings by themselves sometimes or in certain combinations. A single table in ATP 1 is called 'governing groups,' that is, the entire signal that follows the governing group is to be performed according to the 'governor.' The letter 'B' indicates this table, and the second letter (A through Z) gives more specific information. For example, 'BA' might mean 'You have permission to . . .' (do whatever the rest of the flashing light, flag hoist or radio transmission says). 'BZ' happens to be the last item of the governing groups table. It means 'well done'." 
"Bravo Zulu" is defined by the Allied Naval Signal Book (ACP 175 series), an international naval signal code adopted after the North Atlantic Treaty Organization (NATO) was created during 1949. Until then, each navy had used its own signal code and operational manuals. World War II experience had shown that it was difficult or impossible for ships of different navies to operate together unless they could communicate readily, and ACP 175 was designed to remedy this.
In addition to flaghoist and voice radio, use of the term Bravo Zulu has also been extended in contemporary times to include written correspondence and message traffic from senior naval officers in command (i.e., captains and flag officers) to congratulate or otherwise compliment contemporaries or juniors, to include their subordinate crews or commands, for outstanding performance.
- "U.S. Navy: The meaning of 'Bravo Zulu'". U.S. Navy. Retrieved May 18, 2015.