Link farm
A link farm is a group of websites that exist mainly to link to each other or to paying sites, to push up rankings. Google treats it as spam.
A link farm is a group of websites set up mainly to link to each other, or to sites that pay to be included, so that they all look more popular to search engines. The sites usually have thin or copied content, unrelated subjects and pages crammed with outbound links. Some are run as a service: pay a fee and your link appears on hundreds of them.
It's one of the oldest forms of link spam, and one of the easiest for search engines to spot.
Why it doesn't work any more
Google's spam policies list buying links, automated link building and low-quality directory links as link spam, which covers what link farms sell. Since Penguin joined Google's core systems, Google mostly ignores such links. In worse cases, a site can get a manual action for unnatural links, which shows up in Search Console.
A close cousin is the private blog network: a set of sites, often on old expired domains, that one owner uses to link to their own money sites. Google lists expired domain abuse in its spam policies too.
Link farms and AI answers
There's a newer version of the same trick aimed at AI platforms: networks of thin sites that publish lists and "reviews" naming the same brand, hoping an AI platform reads them and repeats the claim. It's the link farm idea with mentions in place of links.
It's a risky bet. For example, if a brand appears only on pages that nobody else links to or reads, an AI answer that does pick one up is quoting a source a reader can check in seconds. If readers notice, it can cost the brand more trust than it gained.
In AnswerOrbit
Sources shows every site the answers used for your prompts, grouped by type. If a cluster of unknown sites keeps showing up in answers that name a competitor, the Clusters tab, which shows sources answers tend to use together, is a good place to look. See Sources.