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.]
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   1. (Arith.) A character [0] which, standing by itself,
      expresses nothing, but when placed at the right hand of a
      whole number, increases its value tenfold.
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   2. One who, or that which, has no weight or influence.
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            Here he was a mere cipher.            --W. Irving.
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   3. A character in general, as a figure or letter. [Obs.]
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            This wisdom began to be written in ciphers and
            characters and letters bearing the forms of
            creatures.                            --Sir W.
                                                  Raleigh.
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   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
      N. W.
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   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.
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            His father . . . engaged him when he was very young
            to write all his letters to England in cipher. --Bp.
                                                  Burnet.
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   Cipher key, a key to assist in reading writings in cipher.
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