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Meaning of monoalphabetic substitution

mon'o•al•pha•bet'ic substitu'tion

Pronunciation: (mon'ō-al"fu-bet'ik, mon"ō-), [key]
— Cryptography. Cryptography.
  1. a system of substitution that uses only one cipher alphabet in a cryptogram so that each plaintext letter is always represented by the same cipher. Cf.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.