An interesting discussion is heating up over at SEO Chat where I am one of the moderators.
The conversation is about LSI, or latent semantic indexing. That's a fancy way to describe how search engines may deliver synonymous results for a query that doesn't contain particular keywords.
The relevancy to SEO directly is almost zero. In fact, so-called SEO firms delivering LSI services are blowing smoke in your face--it's simply a scam.
So what about LSI? What can we take away, what's useful, and what should we ignore?
I won't pretend to be an expert on the subject because, as noted in an early post three years ago, LSI is a complex mathematical exploration initially explored to find alternative ways of processing data (or so is my take).
LSI itself is about math, not about search engines or SEO. Components of LSI, or merely the concepts, ARE used by Google to improve their search results.
Note, I don't think they've taken LSI itself and plugged it into the alogrithm (LSI is more of a concept and not a formula). Rather, they've (Google) used the idea of delivering relevant information based on synonyms, mispellings, alternate spellings, etc., for user queries.
A method is applied to search terms for determining synonyms or other replacement terms used in an information retrieval system. User queries are first sorted by user identity and session. For each user query, a plurality of pseudo-queries is determined, each pseudo-query derived from a user query by replacing a phrase of the user query with a token. For each phrase, at least one candidate synonym is determined. The candidate synonym is a term that was used within a user query in place of the phrase, and in the context of a pseudo-query. The strength or quality of candidate synonyms is evaluated. Validated synonyms may be either suggested to the user or automatically added to user search strings.
Google will use everything at their disposal to improve search. Concepts of LSI are one of them. However, there's nothing that an SEO or web marketing firm can do (other than a wide linking profile which all reputable web markers employ as strategies anyways) to impact 'LSI search'.