A canonical URL, which is often called a canonical tag, is an HTML code that signals to search engines where they should assign a search value for pages with duplicate or remarkably similar content.
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Canonical is a piece of code added to a web page’s HTML head, indicating to Google whether a content piece is unique or duplicated. Original content must canonical to itself, and duplicate content should address the canonical to the original URL source. Canonicals are often used in online stores to retain full search value. Canonicals are also used to avoid duplicate content problems on a site effectively.