A type of monoalphabetic substitution cipher, wherein each letter in an alphabet is mapped to its numeric equivalent, encrypted using a simple function (ax + b) mod m.
Each letter is enciphered with the function (ax + b) mod m, where b is the magnitude of the shift, and m is the length of alphabet. a has to be coprime to m in order for decryption to be possible.
Caesar's cipher is a most known example of affine cyphers, with formula (x + b) mod 26.