The representative URL selected for a group of duplicate or substantially similar pages.
A canonical URL represents content that can be reached through more than one address. The concept becomes useful when several URLs show the same or substantially similar main content. Rather than treating every version as an independent search result, a search engine can select a representative URL. Some duplication is normal and does not automatically mean a site has violated a spam policy.
The address you prefer and the address Google selects are not necessarily the same. A canonical tag is one way to communicate a preference; the canonical URL is the representative address itself. Google evaluates signals and may choose differently. A developer therefore needs to distinguish what the site declares from what an inspection report says Google has selected.
Canonicalization is also different from redirecting a visitor. A duplicate URL may remain accessible while indicating another preferred version. If an old page has genuinely moved and should no longer serve its own content, that is a separate routing decision. Do not apply one preferred URL to unrelated pages simply because you want them all to benefit a main service page.
Takeaway: For genuinely duplicate content, decide which address should represent it and keep your signals consistent. Investigate an unexpected selected canonical before assuming the page merely needs more keywords.
Example
An online stationery shop’s newsletter adds tracking parameters to a notebook product URL. Both the clean and tracked addresses display the same notebook. The team intends the clean address to represent the product, rather than treating each newsletter parameter combination as a separate product page.
What to check
Check both the declared and Google-selected canonical in Search Console where available. Compare the actual main content of the URLs involved. A missing or unexpected selection may require investigation of access, duplication or conflicting signals; changing one tag is not a guaranteed fix.
For more detail, see Google canonicalization explanation.