Pronunciation: (pol'ē-al"fu-bet'ik, pol"ē-), [key] Cryptography.
a system of substitution that mixes together a number of cipher alphabets in a cryptogram so that each plaintext letter is represented by a cipher that repeatedly changes. Cf. monoalphabetic substitution.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.