1. (Of an interface) Having mode
Modeless interfaces are generally considered to be superior because the user does not have to remember which mode he is in.
2. See modal logic
3. In MS Windows programming, A window with the label "WS_MODAL" will stay on the screen and claim all the user-input.
Other windows can only be accessed if the MODAL window is closed.
Such a window would typically be used for an error dialog box to warn the user for something important, like "Critical error, shut down the system and restart".