In cryptology, a code system, as opposed to a cryptosystem, is a system for substituting whole messages, parts of messages, words, or syllables of one language with words or character strings of another, often artificial, language. Meaning codes operate on semantic language units. A cryptological code is a fixed dictionary containing a list of all strings of plaintext and the associated strings of a code text (usually in tabular form). If a “plaintext part” is not in the codebook, it cannot be encrypted.