(TDD) A terminal device used widely by deaf people for text communication over telephone lines.

The acronym TDD is sometimes expanded as "Telecommunication Display Device" but is generally considered to be derived from "Telecommunications Device for the Deaf", although there is some disagreement on this. The deaf themselves do not usually use the term "TDD", but prefer simply "TTY" -- possibly the original term. The ambiguity between this and the other meanings of "TTY" is generally not problematic. The acronym "TTD" is also common.

The standard most used by TDDs is reportedly a survivor of Baudot code implemented asynchronously at 45.5 or 50 baud, 1 start bit, 5 data bits, and 1.5 stop bits. This is generally incompatible with standard modems.

A typical TDD is a device about the size of a small laptop computer (resembling, in fact, a circa 1983 Radio Shack Model 100 computer) with a QWERTY keyboard, and small screen (often one line high, often made of an array of LEDs). There is often a small printer for making transcripts of terminal sessions.

Because of the Stone Age vintage of this technology (the idiosyncrasy of which drives up the expense of individual units), it is thought that TDD standards should transition to use of standard modem line settings (e.g., ASCII 2400-8-N-1). An obstacle to this is the millions of Baudot-only terminals in use (an example of lock-in with a high cruft factor).

Another scenario sees the use of TDDs being replaced by the use of personal computers and talk protocols, presumably over the Internet.