Linux

To add to what @bloodfart wrote, the history of TTYs (or virtual consoles) goes all the way back to the early days of computing and teletypewriter machines.

In the old days, computers were gigantic, super expensive, and operated in batch mode. Input was often provided through punched cards or magnetic tape, and output was printed on paper. As interactive computing developed, the old teletypewriters (aka TTYs) were repurposed from telecommunication, to serve as interactive terminals for computers. These devices allowed operators to type commands and receive immediate feedback from the computer.

With advancements in technology, physical teletypewriters were eventually replaced by electronic terminals - essentially keyboards and monitors connected to the mainframe. The term "TTY" persisted, however, now referring to these electronic terminals.

When Unix came out in the 70s, it adopted the TTY concept to manage multiple interactive user sessions simultaneously. As personal computing evolved, particularly with the introduction of Linux, the concept of virtual consoles (VCs) was introduced. These were software implementations that mimicked the behavior of physical terminals, allowing multiple user sessions to be managed via a single physical console. This was particularly useful in multi-user and server environments.

This is also where the term "terminal" or "console" originates from btw, because back in the day these were physical terminals/consoles, later they referred to the virtual consoles, and now they refer to a terminal app (technically called a "terminal emulator" - and now you know why they're called an "emulator").

With the advent of graphical interfaces, there was no longer a need for a TTY to switch user sessions, since you could do that via the display manager (logon screen). However, TTYs are still useful for offering a reliable fallback when the graphical environment fails, and also as a means to quickly switch between multiple user sessions, or for general troubleshooting. So if your system hangs or crashes for whatever reason - don't force a reset, instead try jumping into a different TTY. And if that fails, there's REISUB.