SSI is when the operating system maintains its state as a single copy of data in memory.
Each system call on a processor can obtain data by load/store to that single copy.
For example, a system that has an individual boot disk that operates using 1, 2, 512 or more processors. The largest number of processors ever to operate under a single system image was 1,024 processors.
In distributed computing, a SSI cluster is a system in which multiple networks, distributed databases or servers appear to the user as a single system. In other words, the operating system environment is shared by all nodes in the system.
Read the rest of the article here