This is a naval signal, conveyed by flaghoist or voice radio,
meaning "well done"; it has also passed into the spoken
and written vocabulary. It can be combined with the "negative"
signal, spoken or written NEGAT, to say "NEGAT Bravo Zulu,"
or "not well done."
There are some "myths and legends" attached to this
signal. The one most frequently heard has Admiral Halsey sending
it to ships of Task Force 38 during World War II. He could not
have done this, since the signal did not exist at that time.
"Bravo Zulu" actually comes from the Allied Naval Signal
Book (ACP 175 series), an international naval signal code adopted
after the North Atlantic Treaty Organization (NATO) was created
in 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 even impossible, for ships of different navies
to operate together unless they could readily communicate, and
ACP 175 was designed to remedy this.
In the U.S. Navy signal code, used before ACP 175, "well
done" was signaled as TVG, or "Tare Victor George"
in the U.S. phonetic alphabet of that time. ACP 175 was organized
in the general manner of other signal books, that is, starting
with 1-flag signals, then 2-flag and so on. The 2-flag signals
were organized by general subject, starting with AA, AB, AC, ...
AZ, BA, BB, BC, ... BZ, and so on. The B- signals were called
"Administrative" signals, and dealt with miscellaneous
matters of administration and housekeeping. The last signal on
the "Administrative" page was BZ, standing for "well
done."
At that time BZ was not rendered as "Bravo Zulu," but
in each navy's particular phonetic alphabet. In the U.S. Navy,
BZ was spoken as "Baker Zebra." In the meanwhile, the
International Civil Aviation Organization (ICAO) had adopted English
as the international air traffic control language. They developed
a phonetic alphabet for international aviation use, designed to
be as "pronounceable" as possible by flyers and traffic
controllers speaking many different languages. This was the "Alfa,
Bravo, Charlie, Delta..." alphabet used today. The
Navy adopted this ICAO alphabet in March 1956. It was then
that "Baker Zebra" finally became "Bravo Zulu."