LSI Latent Semantic Indexing

LSI stands for Latent Semantic Indexing and its a way for a search engine to determine what a web page contains without specifically matching the actual term being searched.  A web page about Fitness  will likely have related words such as Exercise or Health on it.

In effect by placing additional weight on related words in a webpage's content, LSI will lower the value of a page that only matches a specific term and doesn't contain other related terms.

One easy way to do a LSI search is to use the ~ symbol when searching in Google.  Using the above term "~Fitness" resulted in seeing words bolded such as "Exercise", "Health" and so on.

The best way to optimize a website for Latent Semantic Indexing is to create what are known as silos. The top level page would pertain to a particular keyword or phrase and then linking pages would fall under it with relating keywords of the same theme.

Here is a good tool for finding LSI related words
http://lsikeywords.com/