A null cipher
is an ancient form of encryption
where the plaintext is mixed with a large amount of non-cypher material. It would today be regarded as a simple form of steganography
. Null ciphers can also be used to hide ciphertext, as part of a more complex system.
In modern cryptology, the term null cipher is also used to mean choosing not to use encryption at all in a system where various encryption options are offered. This is completely different from the sense above.