The Collaborative International Dictionary of English v.0.48:
Cipher \Ci"pher\, n. [OF. cifre zero, F. Chiffre figure (cf.
Sp.cifra, LL. cifra), fr. Ar. [,c]ifrun, [,c]afrun, empty,
cipher, zero, fr. [,c]afira to be empty. Cf. Zero.]
1. (Arith.) A character  which, standing by itself,
expresses nothing, but when placed at the right hand of a
whole number, increases its value tenfold.
2. One who, or that which, has no weight or influence.
Here he was a mere cipher. --W. Irving.
3. A character in general, as a figure or letter. [Obs.]
This wisdom began to be written in ciphers and
characters and letters bearing the forms of
creatures. --Sir W.
4. A combination or interweaving of letters, as the initials
of a name; a device; a monogram; as, a painter's cipher,
an engraver's cipher, etc. The cut represents the initials
5. A private alphabet, system of characters, or other mode of
writing, contrived for the safe transmission of secrets;
also, a writing in such characters.
His father . . . engaged him when he was very young
to write all his letters to England in cipher. --Bp.
Cipher key, a key to assist in reading writings in cipher.