Acronym for Multiple Document Interface; a container window paradigm that keeps all your documents (in a specific program) within the container of a single application window.
(The acronym is normally not expanded.)
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.
