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TF-IDF is a formula to calculate the importance of a particular term in a specific document, in a collection of a number of documents. It is a more effective way than measuring keyword density to analyze whether or not your content is optimized correctly for search engines. Google relies on the variants of this algorithm to deliver relevant and informative search results within milliseconds. Most SEO packages providers use TF-IdF to optimize your website content such that it gains some good ranking, and thus, traffic.