While I am not sure what the exact acronym wording stands for, I can tell you it is a method for allowing deaf people to communicate by translating voice commands into text type messages, and vice-versa. Both sender and recipient must have TTY mode set. Someone who has a little bit more knowledge on this can correct me if I am wrong. JohnEB
tty is not actually an acronym it stands for Teletypewriter machines which took the place of Telegraph Keys.
TTY machines have been adapted for deaf people who call in to a TTY operator who calls the destination and provides the voice link. the tty operator and the deaf person communicate via the TTy link and the TTya operator relays the message back and forth.
PCs using Modems used to have TTY capability where the deaf guy could use a pc to dial up the TTY operator just like he had a teletypewriter machine.