External Link: An external link is a link which references another domain, which means that it originates outside of the website in question. Commonly, these external links are considered editorial links, or links to another website used to improve a web page's quality. These links typically share common or similar topics or keywords, although using too many links with the same keyword can actually register as spam. Instead, editorial eternal links are designed to provide context and sources while increasing the web page's usability.
Because this most common form of external link is designed to improve a website's quality, it is considered a key quality in improving a site's search ranking. While keywords and other qualities will cause a search engine to crawl the page and use it in a relevant search, a website's quality is what determines where it is listed in the thousands of other search results that have likely also been generated for the same search. When you're updating your website, don't forget to link.