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LSI means Latent Semantic Indexing. It's introduced in 2013 by Google (not sure about the year). Coming to your question, LSI Keywords is a SEO element, which is increasing constantly. LSI Keywords might be a set of some meaningful keywords or a sentence, the both must be based on a main keyword. For example, I've a keyboard/website named "Whatsnew" which is my main keyword, and my LSI Keywords can be "what's new on the internet"/"what's new online shopping", etc. And then include the keywords in keywords meta tag. LSI Keywords helps Google to better understand your website when submitting your website to Google for crawling.