Cipher (Arab, sifr, empty), one of the ten characters used in the notation of numbers by the Arabic system. When it stands by itself it signifies zero. Each cipher placed at the right of a significant figure representing a whole number increases its value tenfold. In decimal fractions every cipher that is placed before a significant figure reduces it to one tenth its previous value. - The term is also used for an enigmatical intertexture of letters, as the initials of a name engraved upon carriages, furniture, plate, seals, or tombs, which it was formerly the custom of tradesmen and citizens to display, much as a coat of arms was displayed by the nobility; also to the monogram or conventional figure by which some artists have designated their names upon their works. - It is also the name of any disguised method of writing, designed to be understood only by the persons who have especially agreed upon the significance of the characters employed. The oldest example of this is the Spartan scytale. When the general of the army departed on any expedition, he took a round wooden staff, called a scytale, with him, leaving another exactly like it with the ephors. When the latter had any communication to make to him, they wound a slip of parchment around the staff, and on this wrote the message.

Being unrolled, only detached and fragmentary letters appeared; but when sent to the general he was able to put it in position upon his staff and read it. The art of secret writing in modern times was long regarded as a branch of magic. When the abbot Trithemius published a treatise on it about 1500, the elector palatine caused a copy of it to be burned, as containing diabolic mysteries. More recent writers on the subject are Giambattista Porta, Vigenere, J. P. Niceron, Kluber (Tubingen, 1809), and Martens (Leipsic, 1851). The chief use of cipher wri-ting since the time of Richelieu has been in diplomatic correspondence, employing various methods, by figures, letters, and catchwords. The introduction of the electric telegraph in recent times has also given occasion for the use of commercial ciphers, by which merchants and bankers transact distant affairs without betraying their secrets.