Latent Semantic Indexing
Search engines create indexes of all of the words on your site. So if your site mentions 'cats' it will be returned in a search for 'cats'. But words with similar meanings (semantics) will also be indexed together, so a page that mentions 'felines' and 'kittens' will be included in the index for 'cats'.
What does this mean for SEO? Previously, good SEO advice included 'keyword stuffing' -- or mentioning a keyword as many times as possible within a page. Now, as indexing gets smarter, keyword stuffing is increasingly punished. In retaliation, SEO specialists have realized that they can get a similar benefit by instead stuffing a page with 'natural' sounding synonyms of the keyword being targeted. This concept is what people mean when they refer to LSI.
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