1.NounA string representing an encoded piece of text.
"Bob cannot get any information out of the codeword since he doesn't know what transmission bases Alice used."
2.NounA word or phrase (or by extension a concept) used to make a (concealed) reference to another word or concept.
3.NounA type of crossword puzzle where the letters of the alphabet are represented by numbers and the solver must identify them by their position and frequency.