Definition = LSI is the most important in the search industry. It means if you are searching keyword then you should choose keyword relevant and similar to topic.
For example, If you are searhing keyword "School Shoes" then we should keyword related to shoes like "black school shoes", "school shoes for boys", "school shoes for girls" etc.
If you choose "party wear shoes" then it'll not come under LSI keyword category.
Latent Semantic Indexing - LSI is a synonym of your targeted keyword which is also search by users. When you search any keyword in google at the end of page you can see the "Search Related" many suggestion its a LSI. If you use a LSI in your main content than you get more traffic and your content is more accurate.
Latent Semantic Indexing (LSI) is a method used by Google and extra main search engines. The contents of a webpage are crawled through a search engine as well as the most common words and idioms are collated and recognized such as the keywords for the page