A teletypewriter (TTY) is a communication device used by people who are deaf, hard of hearing, or have a severe-speech impairment. A TTY consists of a keyboard, display screen, and modem. Messages travel over regular telephone lines. People who don't have a TTY can communicate with a TTY user through a message relay center (MRC). An MRC has TTY operators available to send and interpret TTY messages.
Teletypewriter: also known as a TDD (Telecommunications Device for the Deaf).
Source: U.S. Patent and Trademark Office.