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cipher

(also cypher), n. ~ 1. A technique to convert information (plaintext) into a form that is unintelligible (ciphertext) without knowing the rules (algorithm) to convert it back to its original form. - 2. A message that is so encoded; synonymous with ciphertext. - 3. A key that is used to convert the plaintext to ciphertext. - 4. A design of interlacing initials; also cypher monogram.

Notes: 

Cipher4 monograms frequently appear on the backs of 19th-century cartes-de-visite and cabinet cards.