Cross-linking denotes the mutual linking of two or more websites.
The purpose of cross-linking is mutual support in search engine index ranking. Each internal link provides link power and can affect ranking positively. Both the page being linked to and the page being linked from can benefit from cross-linking. Furthermore, link juice inheritance is intended. PageRank and trust add value to a page and thus serve as motivation for cross-linking.
If the domains involved belong to the same webmaster, the intention behind cross-linking is to strengthen both sites. The idea of two separate websites supporting each other is obvious and legitimate. Caution, however, is advised in implementation.
Cross-linking is a good way to help the parties involved to a better position in rankings. If excessively done, this method can also cause negative effects. There is a risk that Google will consider these websites link networks or even link farms with boundless use of cross-linking because a remarkably rapid and unnatural growth of link popularity is observed. Cross-linking to subpages contributes to this. The building of link networks and link farms are considered a manipulation attempt and may result in penalties being issued by Google. Such penalties vary in degree and can even lead to permanent exclusion from rankings.
From a user perspective, cross-linking can be judged negatively. Matt Cutts answered the question as to whether it is a problem to cross-link your three websites in early 2010 in a video.
He stressed that the webpages involved in cross-linking should be on the same topic. Otherwise, a link does not make sense, because it does not offer visitors further information. (Many) meaningless links at the bottom of a page irritate visitors and cause doubts about the legitimacy of the page. Too many links that are not related to the page in terms of subject may prompt users to leave the page. However, Matt did not say anything about the way the search engines deal with cross-links.
As a general rule, cross-linking can be considered a positive action if used in moderation and sensibly. The following should be noted and is relevant both for visitors and search engines: