Canonical and Canonicalization Glossary
The vocabulary of canonical tags, duplicate URLs and indexing signals, defined in plain English.
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 domain
The one host and protocol a site chooses to be indexed under, for example https://www. rather than http:// or the bare domain, enforced with redirects and matching canonicals. See canonical domain.
Canonical redirect
A permanent (301 or 308) server-side redirect from a non-canonical URL variant, such as the http or non-www version, to the canonical URL. Google rates permanent redirects a strong canonicalization signal. See canonical redirects.
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.
Faceted navigation
Filters such as color, size or price that add parameters to category URLs and can multiply one listing into thousands of crawlable URLs. See canonical tags and URL parameters.
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.
HTTP Link header
A response header that can carry a canonical, written as Link: <URL>; rel="canonical". Used for PDFs and other files that have no HTML head. See canonical in the HTTP header.
Keyword cannibalization
Several pages on one site competing for the same query with the same intent. Google does not penalize it; the usual fix is consolidation with a 301. See keyword cannibalization.
Link relation
The typed relationship a rel attribute declares between two resources, such as canonical, alternate, nofollow or amphtml. See the HTML rel attribute.
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.
Non-canonical URL
A URL that redirects or points its canonical to another URL, or that Google clustered under another URL. Normal to have, but it belongs in no sitemap and no internal link. See non-canonical URLs.
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=amphtml
The link from a regular page to its AMP version. The AMP page links back with rel=canonical, and the pair only works when both annotations exist. See AMP and mobile URLs.
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.
x-default
The hreflang value that names the page for users whose language or region no other alternate targets, often a language selector or a global page. See hreflang tags.
X-Robots-Tag
An HTTP response header that carries the same indexing directives as the robots meta tag, such as noindex, for any file type. See meta robots and X-Robots-Tag.
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.