An indexing method used in search engine algorithms to extract the conceptual content of a body of text by associating the actual words used, with other words or concepts that occur in similar contexts. Used in SEO to increase page rank by using similar words and concepts related to the page text, without resorting to too much repetition of keywords, i.e. keyword stuffing (which lowers page rank).
Ex. If your keyword is lose weight, rather than simply repeating it (more than once per hundred words, which is optimal), you could include related conceptual keywords like diet, exercise, slim, eat healthy, drop pounds, etc. that Google will relate using LSI to increase the SEO value of your article or blog post.