1. Short for TeleTYpewriter, TTY is a communications setup that has a keyboard and a printer. Each time a key is pressed, it is automatically printed to paper. Later, modern TTY machines print to a screen. Teletype machines were available in three different modes: Automatic Send and Receive (ASR) -- Able to send and receive, Receive Only (RO) -- only capable of receiving data and not sending, and Keyboard Send and Receive (KSR) -- TTY with keyboard capable of sending and receiving data.
2. Today, TTY is used to describe a terminal session or line. Users who are using Linux may view a process or which users are logged in by typing a tty, such as ttyx, where x is any numerical value. For example, tty1, tty2, tty3, tty4, tty5, tty6, tty7, and so on.