Canonical (also known as a canonical link element or canonical tag):

An HTML element that is used to avoid duplicate content problems by telling search engines which page of content is the preferred version. It also can be explained as a piece of code that is added into the html head of a webpage to indicate to Google whether a piece of content is original or duplicated from somewhere else. Original content should canonical to itself, and content taken from other places should point the canonical to the original source URL. Canonicals can also be used to avoid duplicate content issues within a website.