/ Dictionary / Index M monoalphabetic substitution: Meaning and Definition of Find definitions for: mon'o•al•pha•bet'ic substitu'tion Pronunciation: (mon'ō-al"fu-bet'ik, mon"ō-), [key] — Cryptography. Cryptography. 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. monoacid monoamine Related Content Daily Word Quiz: secular Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language