A cypher is an algorithm for performing encryption (coding) or decryption (decoding). It is a series of well-defined steps that can be followed as a procedure. To encipher or encode is to convert information from plain text into cipher or code.

In non-technical usage, a 'cipher' often means the same thing as a 'code'; but in cryptography, cyphers are distinguished from codes. One 20th century source gives this explanation: a cypher is "a method in which the basic unit of concealment is the letter.