Link farm

On the World Wide Web, a link farm is any group of web sites that all hyperlink to every other site in the group. In graph theoretic terms, a link farm is a clique. Although some link farms can be created by hand, most are created through automated programs and services. A link farm is a form of spamming the index of a search engine (sometimes called spamdexing). Other link exchange systems are designed to allow individual websites to selectively exchange links with other relevant websites and are not considered a form of spamdexing.