Search Engine Optimization
Canonical URL
A canonical URL is the single preferred address for a page, declared with a canonical link tag, that tells search engines which version to index when the same content is reachable at more than one URL.
The same page can often be loaded several ways: with or without www, with or without a trailing slash, over http or https, or with tracking parameters added. To a search engine, each of those can look like a separate page with duplicate content.
A canonical tag in the page head points every variation at one address. Paired with redirects for the common variations, it keeps ranking signals from being split across duplicates.