Wiktionary
n. (context computing English) a website containing many hyperlinks, especially one designed to increase the index ranking of other websites to search engines
Wikipedia
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 web 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.
Search engines require ways to confirm page relevancy. A known method is to examine for one-way links coming directly from relevant websites. The process of building links should not be confused with being listed on link farms, as the latter requires reciprocal return links, which often renders the overall backlink advantage useless. This is due to oscillation, causing confusion over which is the vendor site and which is the promoting site.