MDI (Multiple Document Interface)
The window model in which an application owns one big frame window and every open document is a child window confined inside it, complete with its own title bar, minimize button and a Window menu offering "Cascade".
Microsoft introduced the model in Windows 2.0 in 1987, and Excel was the first program built on it: spreadsheets sat as windows inside Excel's window and could be moved, stacked and iconified there, but never dragged out onto the desktop. IBM wrote it into CUA in 1989, and in Win32 it became a window class of its own, MDICLIENT, with messages like WM_MDICREATE and a Window menu that drew itself with Cascade, Tile and a numbered list of the children. Minimized documents ended up as small icons along the bottom of the frame. The model gave a multi-document application a single taskbar button and a single menu bar, which was cheap in memory and easy to write.
The Macintosh never had an equivalent, since the menu bar there belongs to the application rather than the window, so every document could be its own window without losing anything. Microsoft itself advised against MDI in the Windows 95 guidelines and moved Word to one window per document with Word 2000. What replaced the child windows was tabs: Opera built its browser as an MDI application with a tab strip on top, and once Firefox and Internet Explorer followed, the tab became the norm for everything from editors to terminals. Qt still ships a QMdiArea for anyone who wants one, and the odd paint program or IDE never stopped.