(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". 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 [Teletype for the deaf?].
The standard most used by TDDs is reportedly a survivor of Baudot code. It uses asynchronous transmission of 1400 Hz and 1600 Hz tones at 45.5 or 50 baud, with one 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. An acoustic coupler connects it to the telephone handset.
With the falling cost of personal computers and the widespread use of Internet talk systems, there is now little reason to use this Stone Age technology.