Single System Image
Single System Image, or SSI, typically refers to the fact that a large computer built from many different components nevertheless runs a single copy of an operating system which controls all of that software. To simplify matters, it would be as if a desktop PC had many additional CPUs and memory cards attached to it, but still ran only one copy of Microsoft Windows or Mac OS X.
By contrast, a cluster of computers will run independent copies of an operating system on each node, and use software to allow programs to be run across all these independent systems as if they were one big computer.
SGI is a premiere designer of highly scalable systems that run a single system image.