Search engine optimization (SEO), and URL canonicalization deals with web content that has more than one possible URL. Having multiple URLs for the same web content can cause problems for search engines – specifically in determining which URL should be shown in search results.

For example:

• http://wikipedia.com
• http://www.wikipedia.com
• http://www.wikipedia.com/?source=asdf

All of these URLs point to the homepage of Wikipedia, but a search engine will only consider one of them as the canonical form of the URL. For SEOs, canonicalization refers to individual web pages that can be loaded from multiple URLs. This is a problem because when multiple pages have the same content but different URLs, links that are intended to go to the same page get split up among multiple URLs. This means that the popularity of the pages gets split up. Unfortunately for web developers, this happens far too often because the default settings for web servers create this problem.