A canonical URL is the preferred URL for a page when multiple URLs serve substantially the same content, signaled via the rel="canonical" link element.
Canonical URLs solve the duplicate-content problem: a single product reachable at /products/x?utm=email, /products/x?ref=affiliate, /products/x?sort=newest, and /products/x should consolidate to one URL that accumulates ranking signals. The canonical declaration tells Google "this is the URL to index and rank."
Emitted as <link rel="canonical" href="https://example.com/products/x"> in the document head. Self-referencing canonicals (a page declaring itself canonical) are encouraged on every page to make the signal explicit. Pagination, faceted navigation, and search-results pages canonical to their base URL to avoid index bloat.
Canonical is a hint, not a directive — Google may choose a different URL as canonical if its signals (links, content, sitemap) disagree with the declared canonical. The strongest canonical signal is consistency across HTML, sitemap, internal links, and external links pointing at the same URL.