Polybius square

Meanings

1. A character code represented by two sets of six lit torches, used to send messages character by character, in a two point visual fire telegraph. The character code is generated by encoding characters in a grid, where each character generates two numbers, uniquely representing each character as a pair of numbers.
2. A grid used for fractionating plaintext characters so that they can be represented by a smaller set of symbols.

Etymology

From Polybius + square (“grid”). It was invented by the Ancient Greek historian and scholar Polybius.