For terms used, like code names but without formal agreement, to communicate something more selectively than direct language would, see Informal code word.
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In communications, a code is a rule for converting a piece of information (for example, a letter, word, or phrase) into another form or representation, not necessarily of the same sort.
Codewords were chosen for various reasons: length, pronounceability, etc. Meanings were chosen to fit perceived needs: commercial negotiations, military terms for military codes, diplomatic terms for diplomatic codes, any and all of the preceding for espionage codes,...
In mathematics, a Gödel code was the basis for the proof of Gödel's incompleteness theorem.
A word that has been assigned a classification and a classified meaning to safeguard intentions and information regarding a classified plan or operation.
In a code, a word that consists of a sequence of symbols assembled in accordance with the specific rules of the code and assigned a unique meaning.
A code may also be used to represent a compressed form of a word, phrase or data.