Null cipher
A null cipher, also known as concealment cipher, is an ancient form of encryption where the plaintext is mixed with a large amount of non-cipher material. Today it is regarded as a simple form of steganography, which can be used to hide ciphertext.

Classical cryptography 
In classical cryptography, a null is intended to confuse the cryptanalyst. In a null cipher, the plaintext is included within the ciphertext and one needs to discard certain characters in order to decrypt the message. Most characters in such a cryptogram are nulls, only some are significant, and some others can be used as pointers to the significant ones.

Here is an example null cipher message, sent by a German during World War I:

Taking the first letter of every word reveals the hidden message "Pershing sails from N.Y. June I".

A similar technique is to hide entire words, such as in this seemingly innocent message  written by a prison inmate but deciphered by the FBI:

Taking only every fifth word, one can reconstruct the hidden text which recommends a "hit" on someone:

Other options include positioning of the significant letters next to or at certain intervals from punctuation marks or particular characters.

Historically, users of concealment ciphers often used substitution and transposition ciphers on the data prior to concealment. For example, Cardinal Richelieu is said to have used a grille to write secret messages, after which the blank spaces were filled out with extraneous matter to create the impression of a continuous text.

Usage 
In general, it is difficult and time-consuming to produce covertexts that seem natural and would not raise suspicion. If no key or actual encryption is involved, the security of the message relies entirely on the secrecy of the concealment method. Null ciphers in modern times are used by prison inmates in an attempt to have their most suspicious messages pass inspection.

See also
 Transposition cipher
 Substitution ciphers
 Acrostic

References

Classical ciphers
Steganography