A tag that tells search engines which URL is the official version of a page, consolidating duplicate signals onto one.
When you have near-duplicate URLs — with and without a trailing slash, with tracking parameters, printer versions — the canonical tag says "this is the official version," so signals consolidate onto one URL instead of scattering.
Two common failures: a self-referencing canonical that points to a slightly different URL than the one that loads (signals scatter), and a cross-page mistake where a page canonicalizes to an unrelated page (telling engines to ignore it). One clear, correct canonical per page is the goal.
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