English
Polybius square
From Polybius + square (“grid”).
The story
It was invented by the Ancient Greek historian and scholar Polybius.
Today it means
noun 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.
Language periods are conventional ranges, not dates for this word.
Etymograph / Polybius square