Hidden message

A hidden message is information that is not immediately noticeable, and that must be discovered or uncovered and interpreted before it can be known. Hidden messages include backwards audio messages, hidden visual messages and symbolic or codes such as a crossword or cipher. Although there are many legitimate examples of hidden messages created with techniques such as backmasking and steganography, many so-called hidden messages are merely fanciful imaginings or apophany.

The information in hidden messages is not immediately noticeable; it must be discovered or uncovered, and interpreted before it can be known. Hidden messages include backwards audio messages, hidden visual messages, and symbolic or codes such as a crossword or cipher. There are many legitimate examples of hidden messages, though many are imaginings.

A backward message in an audio recording is only fully apparent when the recording is played reversed. Some backward messages are produced by deliberate backmasking, while others are simply phonetic reversals resulting from random combinations of words. Backward messages may occur in various mediums, including music, video games, music videos, movies, and television shows.

Backmasking is a recording technique in which a message is recorded backwards onto a track that is meant to be played forwards. It was popularized by The Beatles, who used backward vocals and instrumentation on their 1966 album Revolver. The technique has also been used to censor words or phrases for "clean" releases of songs.

...
Wikipedia