The art of secret writing, characters, or ciphers known only to persons that correspond with one another.
This is from the June 2 calendar page in Forgotten English by Jeffrey Kacirk:
On June 2, 1944, "as the Allies prepared Operation Overload ("D-Day" invasion of Normandy's Omaha and Utah beaches, among others, on June 6), a bit of newspaper minutia raised red flags among the military brass. During the preceding month, it seemed that crossword puzzle's in London's Daily Telegraph were disseminating secret code words to those who knew where to look. Strangely, the names Overload, Omaha, Utah, and mulberry (the nickname of an English-made portable harbor built for D-Day) were all included. Then, on June 2, the name of the naval portion of the assault itself, Neptune, appeared as a solution to "eleven across" from the previous day. British intelligence quickly moved to arrest Leonard Dawe, amild-mannered physics teacher who had written thse newspaper puzzles for the last two decades. But the authorities soon determined that these word choices were simply an incredible coincidence rather than espionage."
I love learning about these little tidbits in history!