Canonical, in the general sense, means the primary value or accepted primary source. In the web industry, this is most known in conjunction with “Canonical URL” or “Canonical Tag”. These both are used as representations to provide and disambiguate between multiple sources that may have the same or similar information.
A “Canonical URL” represents the individual URL that provides a unique set of information. Often many websites have pages that include informational overlap which the search engines may see as “duplication”. If the “canonical URL” value is properly defined by seeing a “canonical tag”, then these repetitive pages can be unified so that the most robust and user-friendly page of the set is presented as the primary page for that information.
A “Canonical tag” is the actual coded element in a website’ssection that is used to signal to a search engine what the primary or base URL/page the information can be found at. This could be used to marry up multiple pages from different site areas or, the most common usage is specific to areas of sites that have multiple pages such as a product category page. In this case, the “canonical tag” is used in conjunction with the ‘prev’ and ‘next’ metatags.