A canonical URL is the practice of choosing one “canonical” URL from among different URLs that point to the same content. This is used so that search engines can prevent content duplication and accurately evaluate the importance of web pages without negatively affecting their search rankings. For example, suppose you have the following URLs: 1. http://example.com/page 2. http://www.example.com/page 3. https://example.com/page 4. https://www.example.com/page Even though all of these URLs point to the same content, search engines may treat them as different pages. The canonical URL uses the HTML <link rel="canonical"> tag to tell search engines which URL is the preferred one.