Nihilist cipher

Etymology
The cipher was originally used by Russian Nihilists in the 1880s to organise terrorism against the tsarist regime.

Noun

 * 1)  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;  any of several improved algorithms based on this.