A link from one website pointing at another external website.
Some webmasters believe in link hoarding, but linking out to useful relevant related documents is an easy way to help search engines understand what your website is about. If you reference other resources it also helps you build credibility and leverage the work of others without having to do everything yourself. Some webmasters track where their traffic comes from, so if you link to related websites they may be more likely to link back to your site.
- Live Search: LinkFromDomain:SEOBook.com – shows pages that my site links at.