I tried to install an nvidia driver and there was an error saying I must not be in an X terminal when doing so. So I did a Ctrl+Alt+F1.
The resulting terminal did not allow the nvidia driver to be installed and showed the same error message. How do I revert back to the Unity GUI?
Ctrl+Alt+F1 to F6 are the virtual consoles provided by the getty/agetty programs.
Ctrl+Alt+F7 is the console where your X server is running. The GUI (Gnome/KDE or any other) runs over X.
So to get back into your GUI window manager: type:
Ctrl+Alt+F7
or
Alt+F7
or
Ctrl+Alt+F8
With 17.10 and newer, the login screen is on virtual terminal 1, and logged-in users' GUI sessions on VT2 and onwards. So, you may need to use Ctrl+Alt+F2 or Ctrl+Alt+F1 instead.
When you are in a virtual console just press Alt+RightArrow or Alt+LeftArrow to move to next/previous virtual console respectively. Console with X is usually seventh.
For completeness, I'll mention the chvt command.
The virtual terminal offers a login prompt, and if you have logged in, you can switch to other VTs:
chvt 7
will probably get you back to the GUI, since it usually runs on VT7, as mentioned in the other answers. This is useful for scripting (for example, when taking a screenshot of LightDM).
You can determine on which terminals you and any other users are logged in with the who command. For me it was tty9, so I had to press Ctrl+Alt+F9.
It has been already mentioned that cycling with Alt+ArrowLeft until you get back to your bellowed desktop also works, especially accross distro's (my Fedora 20, for instance, has GUI on F2), so it's convenient.
There's also a crude method: login and run sudo service lightdm restart, which will restart your session ( so yes, you will be logged out and will get back to login screen ). This answer also takes into account that your login manager might not be lightdm, but the idea is the same.
Some Apple Keyboards require holding the Fn key to complete
Ctrl+Alt+F7 sequence because the top keys have different assignments by default.
