(1) An abbreviation for teletypewriter. Machinery or equipment that employs interactive text based communications through the Transmission of coded signals across the telephone network. TTYs may include, for example, devices known as TDDs (telecommunication display devices or Telecommunication devices for deaf persons) or computers with special modems. TTYs are also called text telephones. (2) A type of machine that allows people with hearing or speech disabilities to communicate over the phone using a Keyboard and a viewing screen. It is sometimes called a TDD.