In user interface design, a modal window is a child window that requires users to interact with it before they can return to operating the parent application, thus preventing the workflow on the application main window. Modal windows are often called heavy windows or modal dialogs because the window is often used to display a dialog box.
An example of a modal dialogue in SharePoint:
Questions tagged modal-dialog regard development, implementation and maintenance of Modal Dialoges in SharePoint.
Reference: Modal Window