In cryptography
Cryptography is the practice and study of techniques for secure communication in the presence of third parties...

, a cipher (or cypher) is an algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...

 for performing encryption
In cryptography, encryption is the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information...

 or decryption — a series of well-defined steps that can be followed as a procedure. An alternative, less common term is encipherment. In non-technical usage, a “cipher” is the same thing as a “code
Code (cryptography)
In cryptography, a code is a method used to transform a message into an obscured form, preventing those who do not possess special information, or key, required to apply the transform from understanding what is actually transmitted. The usual method is to use a codebook with a list of common...

”; however, the concepts are distinct in cryptography. In classical cryptography, ciphers were distinguished from codes.
Codes operated by substituting according to a large codebook
A codebook is a type of document used for gathering and storing codes. Originally codebooks were often literally books, but today codebook is a byword for the complete record of a series of codes, regardless of physical format.-Cryptography:...

 which linked a random string of characters or numbers to a word or phrase.