Canonical tag
Short definition
A canonical tag is an HTML marker that tells a search engine which version of several near-identical pages is the one to index.
On an online shop, the same product can appear at dozens of addresses because of colour options or filter parameters. A canonical tag tells the search engine 'this is the real page', so it knows which version to index and which one should receive the ranking credit.
Leave the tag out or set it wrongly and the search engine makes its own choice, which often means the wrong page rises or ranking strength gets split across near-duplicates. The same issue appears with www versus non-www, or http versus https.
A canonical tag is often confused with a redirect (301); a redirect physically moves both the visitor and the search engine to a different address, whereas a canonical tag leaves the old page reachable and only offers the search engine a suggestion. Engines usually follow that suggestion but are not obliged to; if other signals on the site disagree, a different page can still be chosen as the true version.
Why it matters
Duplicate content confuses a search engine about which page to trust and splits its ranking strength. A correct canonical tag keeps everything a page has earned in one place.
Illustrative example
A flooring manufacturer added canonical tags to its product variants, and while fewer pages got indexed overall, the main product page climbed in ranking.
