Canonical & Canonicalization Glossary
The vocabulary of canonical tags and URL canonicalization, defined.
Plain-English definitions of the canonical tag, canonicalization, and the related terms that come up alongside them. Each entry links to the fuller explanation where there is one.
301 redirect
A permanent, server-side redirect from one URL to another. It is a strong canonicalization signal, stronger than the advisory canonical tag. See canonical tag vs 301 redirect.
Alternate page
A page that correctly points its canonical at another URL and is intentionally not indexed on its own. See alternate page with proper canonical tag.
Canonical tag
The rel="canonical" link element placed in a page's <head> that names the preferred URL to index when the same content is reachable at more than one address. See what is a canonical tag.
Canonical URL
The single preferred address a search engine indexes and ranks for a set of duplicate or similar pages. See what is a canonical URL.
Canonicalization
The process of choosing one preferred URL from several that serve the same or similar content, so search engines index and rank a single version. See canonicalization in SEO.
Crawl budget
The number of URLs a search engine will crawl on a site in a given period. Duplicate parameter URLs waste it, which is one reason canonicalization matters at scale.
Cross-domain canonical
A canonical tag that points to a URL on a different domain, used in syndication. Google now de-emphasizes it in favor of noindex. See cross-domain canonicals.
Duplicate content
Substantive content that appears on more than one URL. It is not a classic penalty; search engines consolidate the duplicates to a canonical URL. See duplicate content and canonical tags.
Faceted navigation
Filter and sort controls on category or search pages that generate many parameter URLs for the same set of products, a common source of duplicate content on ecommerce sites.
Google-selected canonical
The URL Google actually chooses to index for a page, which can differ from the one you declared because the canonical tag is a hint.
hreflang
An attribute that tells search engines which language or region a URL targets, so the right version is served to the right audience. It must stay consistent with self-referencing canonicals. See canonical tags vs hreflang.
HTTP Link header
A way to declare a canonical for non-HTML files such as PDFs by returning Link: <url>; rel="canonical" as a response header instead of a tag in the markup.
noindex
A robots directive telling search engines not to index a page. It conflicts with a canonical tag when both are applied to the same URL. See canonical and noindex.
Page indexing report
The Google Search Console report (formerly Index Coverage) that shows which pages are indexed and why others are not, including the canonical statuses.
Pagination
Splitting a series across numbered pages. Each paginated page should carry a self-referencing canonical; Google stopped using rel=prev/next for indexing in 2019. See canonicals and pagination.
rel=canonical
The HTML attribute value that marks a link element as a canonical reference. It is a hint search engines consider, not a directive they must obey. See the syntax reference.
Self-referencing canonical
A canonical tag that points a page at its own URL. It is the recommended baseline for pages that should stand on their own. See self-referencing canonical tags.
URL parameter
A query-string value appended to a URL (for example ?color=blue or ?utm_source=…) that often creates a duplicate of the clean URL and needs canonicalizing.
User-declared canonical
The canonical URL you specify for a page through rel="canonical", redirects, or your sitemap. See user-declared vs Google-selected canonical.
XML sitemap
A file listing a site's URLs for search engines. It should contain only canonical, indexable URLs; listing non-canonical URLs sends a mixed signal.