Dialog windows you use to display critical information to the user are, in most cases, displayed modally. A modal window (form) is the one where the application can’t continue to run until the modal window is closed. Delphi’s ShowMessage, InputBox and MessageDlg, for example, display a modal form to the user waiting for some action. Your custom dialogs are displayed using the ShowModal method of a form.
Dimmer – a device for varying the brightness of light
To emphasize the importance of a modal form and the information it presents, you could gray out the main form of the application when the modal form is activated.