1. (Of an interface) Having modes
. 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".