An Acronym with two meanings.

Server Side Includes
A rudimentry language to be used with HTML pages for inclusion of other files and/or some very basic processing, achieving a crude form of dynamic WebPage generation. For very small sites, SSI can be a completely sufficient form of a ContentManagementSystem.

Single System Image
It is used to differentiate between machines such as the SGIAltix and clusters like Beowulfs. An SSI machine runs one copy of the OperatingSystem, and each node can access every other node's memory directly without doing anything special.