The shortcut shown in the Apple menu for the Force Quit menu item is ⌥⌘⎋. (The last character is the Unicode Broken circle with northwest arrow character.)
What keys should I use as shortcut of that menu item?
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Sign up to join this communityTo be concise: the symbol you're asking about, the circle with the arrow escaping from it (that's the way I remember what it means), is the Apple way of saying 'The ESC Key'.
So the menu item ⌥⌘⎋ is option+command+esc pressed simultaneously.
The Escape key.
I don't know why Apple use this non-standard symbol, but they always have...
Here is a full list of all of the symbols that Apple use.
As an extension to Ian's answer, here's a list of all the keys:
As correctly pointed out by Ian, the ⎋ is the Escape key located on the upper left side of your keyboard.
You can find a list of all the keyboard icons, terminology, and HTML entities over here on Meta . That icon ⎋ is the Escape key. You can find it in the very top left-hand corner of your keyboard.