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Meaning of polyalphabetic substitution

pol'y•al•pha•bet'ic substitu'tion

Pronunciation: (pol'ē-al"fu-bet'ik, pol"ē-), [key]
— Cryptography. Cryptography.
  1. 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.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.