The preferred version of a URL when multiple URLs show the same or similar content. Tells search engines which version to index, preventing duplicate content penalties.
A canonical URL is the definitive, preferred URL for a piece of content when multiple URLs could display the same or similar content. It's declared using a canonical tag in the page's HTML: <link rel='canonical' href='https://yoursite.com/preferred-url'>. This tells search engines: 'even if you find this content at other URLs, index and rank this specific version.'
Duplicate content is a common technical SEO problem. The same page can be accessible at multiple URLs for various reasons: HTTP vs. HTTPS, www vs. non-www, trailing slash vs. no trailing slash, URL parameters (like ?utm_source=newsletter), and session IDs. Without canonical tags, search engines may split ranking signals across multiple versions, diluting the value of each. Canonical tags consolidate these signals onto a single preferred URL.
Canonical tags are especially important for e-commerce sites (where the same product might appear in multiple category paths) and for websites with CMS-generated pages that include parameter-based URLs. For simpler service business sites, canonical tags still matter — especially ensuring that the www and non-www versions of the site both point canonically to the same preferred domain.
Canonical tags are not a redirect — the non-canonical pages remain accessible, they just cede their ranking credit to the canonical version. They're also not guaranteed to be followed by search engines, which will use their own judgment if the canonical tag seems wrong. For this reason, canonical tags should complement (not replace) proper URL structure and 301 redirects where appropriate.