In cryptography, padding is any of a number of distinct practices which all include adding data to the beginning, middle, or end of a message prior to encryption. In classical cryptography, padding may include adding nonsense phrases to a message to obscure the fact that many messages end in predictable ways, e.g. sincerely yours.
Classical cryptography
Official messages often start and end in predictable ways: My dear ambassador, Weather report, Sincerely yours, etc. The primary use of padding with classical ciphers is to prevent the cryptanalyst from using that predictability to find known plaintext that aids in breaking the encryption. Random length padding also prevents an attacker from knowing the exact length of the plaintext message.
A famous example of classical padding which caused a great misunderstanding is "the world wonders" incident, which nearly caused an Allied loss at the World War II Battle off Samar, part of the larger Battle of Leyte Gulf. In that example, Admiral Chester Nimitz, the Commander in Chief, U.S. Pacific Fleet in WWII, sent the following message to Admiral Bull Halsey, commander of Task Force Thirty Four (the main Allied fleet) at the Battle of Leyte Gulf, on October 25, 1944:
Halsey's radio operator mistook some of the padding for the message and so Admiral Halsey ended up reading the following message: