An Acronym for Multiple Document Interface. (This acronym is normally not expanded.)

It refers to an interface where the app has a container window that keeps all document windows inside.

Used in MicrosoftOffice up until version 2000, where they decided to go back to SDI? and really confuse everyone. (The reason is that Windows XP supports grouping of taskbar items.) Which I actually find a good reason in itself. It's always good to elevate common interface elements to a higher level. --AristotlePagaltzis

See an example at WikiPedia:Multiple_Document_Interface.