A link element in a page's head that names the definitive URL among duplicates or near-duplicates, so ranking signals consolidate onto one address. Google treats it as a hint rather than a strict rule, and a sitemap that disagrees with the pages' canonicals is largely ignored.
Duplicate content confuses search engines: if two pages are nearly identical, they do not know which one to rank. The canonical tag tells Google which version is the master copy, but because it is a hint, the rest of the site has to agree with it.
Keep URLs short and clean, set a canonical on original pages, and point internal links and the XML sitemap at the same canonical URL. On multilingual sites, hreflang tags tell search engines which language version to show each user, which prevents canonicalisation problems between the versions.
Example
A product page can be reached at /shoes and at /shoes?utm_source=newsletter. Both copies declare <link rel="canonical" href="https://example.com/shoes">, so links pointing at either copy count toward /shoes.
Related terms
Where seodada covers it
The free URL checker shows the canonical a page declares, its status code, any redirect chain and whether robots rules allow indexing. The technical SEO guide covers duplicate URLs and canonical tags in depth.