Co-citation: A co-citation is essentially a link from another page that links to your site along with your competitors' websites. These links help search engines establish what sort of community your page belongs to, so if you run a blog on fashion, you might be linked on a more authoritative site with other fashion blogs.
Because of the ways search engines function, they will often believe that links appearing near one another are related. That co-citation is determined by the frequency in which those two or more sources are cited near one another.
Google's algorithm can look at the co-citations for a particular website in order to determine the page's rank in a search query. The quality of the links used is also a way for Google and other search engines to rank a page within a search. In other words, quality co-citations from trusted sources will fare better than a citation from an obscure website.
Co-citation can also refer to citations done in any sort of document type. For instance, academic papers that all cite a particular source can help boost that source's credibility. In these circumstances, however, cronyism is an issue when several documents cite the sources belonging to friends and colleagues. The same can sometimes be true of co-citations appearing on the internet.