Term
Canonical URL
A canonical URL is the main version of a page when the same or very similar content is available at more than one address.
What it means
Search engines use canonicalization to choose which version to index as the representative page. A canonical tag lets you suggest a preferred URL, although Google may select a different version.
Example
A product page is available at /coffee/ and /coffee/?campaign=email. Both show the same product, so the canonical tag can point to /coffee/ as the preferred address.
Keep in mind
A canonical tag does not send visitors to another page. A redirect does. Use internal links and sitemaps that agree with your preferred URL.