A misspelling of "SSI", Spectral Similarity Index, another color accuracy metric.
Basically the industry figured out how to win at the CRI game without actually creating the same underlying spectral distribution of light. So they same up with another metric to try to optimize called SSI (also TLCI, etc.) SSI is mostly relevant in the digital cinema space, where the observer is a digital camera, not a human eye, as they can't be tricked the same way because they have different underlying RGB spectral sensitivities.
