xml sitemap
A Sitemap index is an XML file that lists multiple XML sitemap files. The XML format of a Sitemap index file is very similar to the XML format of a Sitemap file. It allows webmasters to include additional information about each XML sitemap (when it was last updated). After creation of a Sitemap index file webmasters can notify search engines about the index file, and the other XML sitemaps that are included in the Sitemap index file will be automatically notified too.