The story
The cipher was originally used by Russian Nihilists in the 1880s to organise terrorism against the tsarist regime.
Etymology from Wiktionary, CC BY-SA 4.0.
Nihilist cipher
Today it means: A symmetric cipher in which a Polybius square using a mixed alphabet is used to convert both the plain text and a keyword to a series of two-digit numbers, which are then summed in the normal way to get the ciphertext, with the key numbers repeated as required; (by extension) Any of several improved algorithms based on this. Full definition →