Glossary
Canonical tag
A canonical tag is a line in a page's HTML that names the preferred URL for that page, so search engines know which version to show and count.
One page can be reachable on more than one address: with or without a tracking parameter, with or without a trailing slash, over http and https. To a search engine those can look like separate pages with the same content.
The canonical tag, a link element in the page's head with rel="canonical", says which address you consider the main one. Usually that is the page's own URL. It is a strong hint, not a command: search engines may choose another version if other signals disagree.
A canonical that points to another domain tells search engines that the content belongs there. That is correct for syndicated content you want credited elsewhere, and a mistake almost everywhere else.
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