Canonical URL definition
Canonical URL:
Many content management systems are configured with errors which cause duplicate or exceptionally similar content to get indexed under multiple URLs. Many webmasters use inconsistent link structures throughout their site that cause the exact same content to get indexed under multiple URLs. The canonical version of any URL is the single most authoritative version indexed by major search engines. Search engines typically use PageRank or a similar measure to determine which version of a URL is the canonical URL.
Most glossary items from Aaron Wall's SEO Book.
