A null cipher is an ancient form of encryption where the plaintext is mixed with a large amount of non-cipher 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.

An example follows (Kipper 9):

News Eight Weather: Tonight increasing snow. Unexpected precipitation smothers eastern towns. Be extremely cautious and use snowtires especially heading east. The [highway is not] knowingly slippery. Highway evacuation is suspected. Police report emergency situations in downtown ending near Tuesday.

Taking the first letter in each word successively yields the real message: "Newt is upset because he thinks he is President."

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, an option some software offers for testing/debugging. This is completely different from the sense above.

Reference

Kipper, Gregory Investigator’s guide to steganography 2004 CRC Press LLC