(Telecommunications Device for the Deaf/TeleTYpewriter) A user terminal with keyboard and printer or keyboard and screen used by the hearing and speech impaired. Introduced in 1964, the device contains a modem and is used over a standard analog phone line. If a recipient does not have a corresponding terminal device, TDD/TTY users dial a relay service composed of operators who receive the typed messages, call the recipients and speak the messages to them. The operators also type the responses back to the TDD/TTY user. See VRS.