Page "Code" Paragraph 8

Using terms from formal language theory, the precise mathematical definition of this concept is as follows: Let S and T be two finite sets, called the source and target alphabets, respectively.

A code is a total function mapping each symbol from S to a sequence of symbols over T, and the extension of M to a homomorphism of into, which naturally maps each sequence of source symbols to a sequence of target symbols, is referred to as its extension.