polyalphabetic substitution

American  

[pol-ee-al-fuh-bet-ik suhb-sti-too-shuhn, pol-ee-, ‐-tyoo-shuhn] / ˈpɒl iˌæl fəˈbɛt ɪk ˌsʌb stɪˈtu ʃən, ˌpɒl i-, ‐ˈtyu ʃən /

noun

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.

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Etymology

Origin of polyalphabetic substitution

1935–40; poly- + alphabetic ( def. )