A canonical URL is the web address a search engine selects as the most representative of a set of pages with duplicate content; choosing it is called canonicalisation.
The link relation behind the term was defined by the Internet Engineering Task Force (IETF), the body that develops internet standards. In RFC 6596, an informational document published in April 2012, a 'canonical' link designates one web address as preferred over resources with duplicate content.
Google's documentation on canonicalisation treats a site's stated preference as a hint, not a rule: a site can indicate which URL it prefers, but the search engine may choose a different page for various reasons. It lists three methods of indicating the preference, ranked by how strongly they can influence the choice. Redirects and rel="canonical" link annotations are strong signals, while inclusion in a sitemap is a weak one. None of them is required, although a rel="canonical" link on the canonical page itself, known as a self-referential canonical, is recommended.
In our client work the subject comes up in multilingual shops. For Battle-Merchant we manage canonicals across the shop's language versions; at Cold Steel, the canonical of each brand landing page Landing page As defined in the Google Ads glossary, a landing page is the web page people arrive at after clicking an ad. Read in the glossary → is managed individually in all four of its language versions.
Where the term appears
See also
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SEO
SEO, or search engine optimisation, is the work of helping search engines understand a site's content and helping people find the site through search and decide whether to visit it.
E-commerce
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hreflang
The hreflang attribute gives the language of a linked page and lets a site tell search engines which pages are localised versions of the same content.
E-commerce
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Landing page
As defined in the Google Ads glossary, a landing page is the web page people arrive at after clicking an ad.
E-commerce
Sources
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RFC 6596: The Canonical Link Relation (April 2012) (opens in a new tab)
IETF / RFC Editor accessed
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Was ist Kanonisierung? (opens in a new tab)
Google Search Central accessed
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Kanonische URL mit rel="canonical" und anderen Methoden angeben (opens in a new tab)
Google Search Central accessed