The term Canonical is derived from mathematics and means a URL in its simplest or standard form. It is widely used within SEO circles. For example, a home page could have multiple URLs:
http://www.mysite.com/ http://mysite.com/ http://www.mysite.com/index.php
These are different URLs as far as a search engine is concerned as technically a Web server could return different content for each. However, many web servers are configured to return exactly the same content:
index.html
In this case, we should pick one version of the URL, the canonical form, and use this both internally and externally. All other forms should use an HTTP 301 permanent redirect to send search engine robots (and users) to the correct version.