A link to a site outside of your site.
Outbound links send visitors away from your web site. Attitudes towards outbound links vary considerably among site owners. Some site owners still link freely. Some refuse to link at all, and some provide links that open in a new browser window.
Opponents of outbound linking argue that it risks losing time and money from site visitors. This can be a large risk if a site is facing high customer acquisition costs.
Proponents argue that providing high quality references actually enhances the value of a site and increases the chance of return visitors.
see also: SEO Tools