Text Telephone / Teletype Terminal / TeleTYpewriter
Telecommunication Device for the Deaf
A TTY is a special device that lets people who are deaf, hard of hearing, or speech-impaired use the telephone to communicate, by allowing them to type text messages. A TTY is required at both ends of the conversation in order to communicate.
It can be used with both land lines and cell phones. Unlike text messaging, it is designed for synchronous conversation, like a text version of a phone call.
A modern digital cell phone must support a special digital TTY mode in order to be compatible with a TTY device.
See: Digital TTY/TDD
TDD can also stand for Time Division Duplex, a method for general two-way radio communication, unrelated to hearing-impaired applications.