Way back when computers were just starting to appear, there was an old-timey device called the teletypewriter (TTY). It allowed an operator to send a text message across a wire. In modern day usage, a TTY is a default terminal used to interface directly with your operating system. Linux borrows this terminology from UNIX.
Terminal applications like GNOME Terminal, Alacritty, and Guake are actually better known as terminal emulators, because they perform the job of a TTY on top of your desktop environment. The TTY always sits behind any graphical interface. This distinction is exactly why you may be able to access your TTY using Ctrl + Alt + F3 even when you cannot open the terminal on your desktop.