SEO
Canonical URL
The single authoritative address you declare for a page's content, so search engines consolidate duplicates and rank one URL instead of splitting signals.
A canonical URL is your answer to a question every site eventually raises: when the same content is reachable at more than one address, which one counts? Trailing slash or not, http or https, with tracking parameters or without, the content is identical but the URLs are not. Left unresolved, search engines treat each variant as a separate page and split ranking signals between them.
The rel="canonical" tag declares the authoritative version. Done correctly, every variant points at one clean URL, links and equity consolidate there, and the crawler stops wasting budget on duplicates. Done wrong, it is one of the quietest ways to hurt a site: canonicals that point at redirects, at 404s, or at the wrong page entirely.
Canonicals earn their keep most during migrations and redesigns, when every URL on the site is changing at once. A migration that preserves canonical discipline keeps its rankings; one that improvises loses them. It is a small tag with outsized consequences, which is why it appears in every audit we run.