What is Canonical URL?
A canonical URL is the single address you declare as the official version of a page, using a rel="canonical" tag, so search engines consolidate ranking signals on one URL instead of splitting them across near-identical copies.
Also called: canonical tag, rel=canonical, duplicate content tag.
Websites accidentally produce duplicates constantly: with and without www, http and https, a trailing slash or not, tracking parameters glued on by an ad campaign, a printer-friendly variant. To a search engine each of those is potentially a separate page competing with itself.
The canonical tag settles it. You nominate one URL, every duplicate points at it, and the links and authority earned by all of them accrue to that one address. Get it wrong — a canonical pointing at your homepage from every page, a common CMS misconfiguration — and you can de-index most of your site while believing you are tidying it.
For a small business the risk is usually mundane rather than dramatic: a Facebook campaign sends traffic to `?fbclid=…`, and the version people actually link to isn't the version Google decided to rank.
What to actually do about it
- Give every page a self-referencing canonical pointing at its clean URL.
- Pick one host — either www or non-www — and redirect the other permanently.
- Force HTTPS, and keep trailing slashes consistent site-wide.
- Never canonicalise a whole section to the homepage.
- Keep canonicals absolute, not relative.
Koo sets a self-referencing canonical on every page, enforces one primary domain with redirects from the rest, and strips tracking parameters from the canonical, so duplicate versions never compete with each other.
Related terms
Meta description
A meta description is a short summary in a page's HTML — best kept under about 155 characters — that search engines often display beneath the page title, acting as the advert for the click.
XML sitemap
An XML sitemap is a file listing the URLs on a site that you want search engines to crawl, along with when each was last changed, submitted through Search Console so new and updated pages are found quickly.
robots.txt
robots.txt is a plain-text file at a site's root that tells crawlers which paths they may or may not request. It controls crawling, not indexing, and it is a public request rather than a security measure.