I can't for the life of me find where it's specified. Oddly I don’t think I’ve had this problem before in unity. When editing in emacs, the alt+w short-cut is vital and every time I press it I get the "type your command prompt" instead.
2 Answers
Your problem is the alt key. Any combination using it will trigger the HUD. You can use the package compizconfig-settings-manager.
Step 1 : Install compizconfig-settings-manager
by running the following command in a terminal:
sudo apt-get install compizconfig-settings-manager
Step 2 : After installing it, open it and go to "Ubuntu Unity Plugin". Click on the button for the option Key to show the HUD.
Step 3 : Click on Grab key combination and press any new key combination and press enter. After that, alt should no longer trigger the HUD.
Also, I believe this has been already explained more clearly :

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Or you can go to System Settings --> Keyboard --> Shortcuts as explained in this answer. – HelloGoodbye Oct 30 '17 at 10:10
AFAIK, Alt+W is not a shortcut in unity. To get the "Type your command here prompt". You press the Alt button. I think there is a gap in your typing the Alt and W keys. Try pressing them together faster

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My case is the opposite. I need to type Alt+W very slowly (i.e. press alt, then keep the key pressed for a while, then press W) in order not to trigger unity. I will change the HUD key then. Perhaps Ubuntu does not expect any of its users use emacs... – Yi Wang Jul 04 '14 at 07:31