Etymology

14th century. From cyfre, cyffre (French chiffre), ultimately from (sifr) ‘zero, empty’, from (safara) ‘to be empty’. Compare zero.

Alternative spellings

cypher, less common than cipher but still in use in English. see The Ultra Secret by Winterbotham, the cypherpunk movement, and consider the Royal Navy's series of Cyphers (Nr 1, Nr 2, Nr3, ...) before and into WWII.

Noun

1. A numeric character.
2. A method of transforming a text in order to conceal its meaning.

   The message was written in a simple cipher. Anyone could figure it out.

3. A cryptographic system using an algorithm that converts letters or sequences of bits into ciphertext.
4. Ciphertext; a message concealed via a cipher.

   The message is clearly a cipher, but I can't figure it out.

5. A grouping of three digits in a number, especially when delimited by commas or periods:

   The probability is 1 in 1,000,000,000,000,000--a number having five ciphers of zeros.

6. A design of interlacing initials: a decorative design consisting of a set of interlaced initials.
7. A fault in an organ valve which causes a pipe to sound continuously without the key having been pressed.
8. A hip-hop jam session http://www.rapdict.org/Cipher
9. The path (usually circular) shared cannabis takes through a group, an occasion of cannabis smoking.
10. Someone or something of no importance.

    You are just a cipher to me.

11. Zero.

Verb

1. (regional and/or dated) To calculate.

   I never learned much more than how to read and cipher.