Latent Semantic Indexing [LSI] is a mathematical method to analyze information from a document (such as a web page) and produce a set of terms and concepts represented in that document. LSI has been around since the 1980's and was an early version of machines trying to figure out what information was contained in a document so it could be indexed properly and retrieved in searches for similar terms and concepts. So that a document about "arachnids" might show up in a search for "spiders."
There is some controversy over whether Google uses LSI or not.
