Link spam
Google's spam policy category covering links intended to manipulate rankings: buying or selling links, excessive exchanges, automated link…
LinksLink exchange
Reciprocal links
Two sites linking to each other; unremarkable when it reflects a real relationship, a link spam violation when exchanges are excessive or systematic.
In full
Google's policy targets 'excessive link exchanges' and 'partner pages exclusively for the sake of cross-linking'. Natural reciprocity is common — suppliers, partners, co-authors — and is not a problem. The signal that trips systems is scale and pattern: many exchanges arranged for ranking purposes, often coordinated in groups or through outreach networks.
Example
A group chat of forty site owners agrees to link to each other's commercial pages every month; the links stop passing value.
Related terms
Link farm
A group of sites that link to each other extensively for the sole purpose of inflating link counts, with no editorial reason for the links.
Link building
The practice of acquiring links from other websites, ranging from editorially earned citations to schemes that violate Google's link spam…
Link profile
The aggregate characteristics of all links pointing to a site: source diversity, anchor text distribution, follow ratio, topical relevance…
Guest posting
Writing articles for other publications; legitimate as an audience and authority tactic, but a link spam violation when done at scale…