A canonical URL is the definitive version of a web page's address that a site owner designates as the preferred URL for search engine indexing. When the same content is accessible through multiple URLs (due to query parameters, session IDs, or protocol variations), the canonical tag tells search engines which version to treat as authoritative.
The canonical tag is implemented as a link element in the HTML head: <link rel="canonical" href="https://example.com/page">. This signals to search engines that regardless of which URL variation a crawler discovers, the specified canonical URL should receive the indexing priority and link equity. SEO fundamentals books on Amazon explain implementation details.
In URL shortening, canonical URLs interact with short URLs in an important way. The destination page should always specify its own canonical URL, not the short URL. This ensures that search engines index the original page rather than the short URL, which is the desired behavior since short URLs are meant for sharing, not for search engine ranking.
Common mistakes include setting the canonical URL to a relative path instead of an absolute URL, having conflicting canonical tags across different page versions, and forgetting to update canonical URLs after site migrations. Technical SEO books on Amazon cover audit procedures.