Canonical URL
A canonical URL is the preferred version of a web page when multiple URLs serve similar or identical content. The rel='canonical' tag tells search engines which URL to index, preventing duplicate content issues across domains or [[subdomain|subdomains]].
Example
If example.com and www.example.com serve the same page, a canonical tag on both pointing to example.com consolidates SEO signals.
Referenced in Guides
Domains & SEO