MDI stands for multiple document interface, more information and an explanotion can be found on wikipedia. Multi document interface is the opposite of singe document interface.

Multiple Document Interface.....................by shiv

The multiple-document interface (MDI) allows you to create an application that maintains multiple forms within a single container form. Applications such as Microsoft Excel and Microsoft Word for Windows have multiple-document interfaces.

SDI is a Single Document Interface. As opposed to MDI which is a Multiple Document Interface. the difference is solely in the number of documents that can be opened at one time in each instance of the application. With SDI only one document can be open whereas with MDI many documents can be open.

MDI (Multiple Document Interface) is a Microsoft Windows programming interface for creating an application that enables users to work with multiple documents at the same time. Each document is in a separate space with its own controls for scrolling. The user can see and work with different documents such as a spreadsheet, a text document, or a drawing space by simply moving the cursor from one space to another. An MDI application is something like the Windows desktop interface since both include multiple viewing spaces. However, the MDI viewing spaces are confined to the application's window or client area . Within the client area, each document is displayed within a separate child window . MDI applications can be used for a variety of purposes - for example, working on one document while referring to another document, viewing different presentations of the same information, viewing multiple Web sites at the same time, and any task that requires multiple reference points and work areas at the same time