Polybius square

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

Polybius square (plural Polybius squares)

1. (telecommunications, historical) 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. (cryptography) A grid used for fractionating plaintext characters so that they can be represented by a smaller set of symbols.

(telecommunications):

• semaphore
• Morse code