Canonical Tag
An HTML link element that tells search engines which version of a page is the "master" copy to index.
A canonical tag (<link rel="canonical" href="...">) is placed in the <head> of a page to point to the preferred URL when duplicate or near-duplicate content exists across multiple URLs. It tells Googlebot: "Even though this page is accessible at several URLs, only count this one."
Common use cases include product pages accessible through multiple category paths, paginated content, HTTP vs HTTPS duplicates, and pages with session IDs or tracking parameters in the URL. Google treats canonicals as a strong hint — not a directive — so it may override them if it determines another URL is more appropriate.
Self-referencing canonicals (where a page canonicalises to itself) are a best practice for all pages, not just duplicates. This prevents any parameter-appended version of your URL from accidentally being indexed.