Canonical URL
Definition
A canonical URL is the preferred address for a piece of content when duplicates or near-duplicates exist, signaled mainly via rel=canonical.
Full definition
Sites often have the same content under multiple URLs (parameters, trailing slashes, mirrors). A canonical tag tells search engines which version to treat as primary for indexing and ranking signals.
Clear canonicals help both traditional SEO and AI systems that may encounter multiple URLs for the same entity page. See Sitemap.