What is a canonical URL?
The one address Google should treat as the original when the same content is reachable at several. It stops a site competing with itself.
The same content often turns up at more than one address without anyone meaning it to. A product might appear under two categories. A page might be reachable with and without a tracking code on the end, or with and without “www”. To you they are one page. To Google they can look like several pages saying the same thing.
A canonical URL tells Google which of those addresses is the original, the one to show in results and to give the credit to. It is set with a small tag in the page’s code pointing at the preferred address.
One thing worth knowing is that Google treats it as a strong suggestion rather than an instruction. If the canonical points somewhere that does not make sense, or conflicts with other signals such as the sitemap or internal links, Google may pick a different address itself. Getting it right means keeping every signal pointing the same way, which is part of technical SEO.
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