Acronym for Silicon Graphics Inc
SGI are well known for their really nifty machines with very pretty graphics (oh, and they look cool too). In the desktop environment, much of this has eroded of late now that Intel machines have got stupidly advanced graphics cards.
However, for things like realtime HD video editing, there is nothing that can touch an SGI box. The I/O and scalability capabilities of the high end machines (Origin, Onyx) are stupid. Think 1024-way SSI machines and enough I/O to backup 10TB/hour to tape.