Link pointing to one website from another website.
Most search engines allow you to see a sample of links pointing to a document by searching using the link: function. For example, using link:www.seobook.com would show pages linking to the homepage of this site (both internal links and inbound links). Due to canonical URL issues www.site.com and site.com may show different linkage data. Google typically shows a much smaller sample of linkage data than competing engines do, but Google still knows of and counts many of the links that do not show up when you use their link: function.