The “master version” of a page on your website that you want search engine crawlers to find. As you launch marketing campaigns and track users, information gets added to the end of your unique page URLs. From the crawlers’ point of view, this means you have multiple versions of the same page without a clear indication of which one is “right.” When you set up a canonical URL in the page’s HTML, you tell search engines which version of the page you want visitors to see.