A Private Blog Network (PBN) is a network of websites, typically created for the purpose of manipulating search engine rankings. A PBN is made up of multiple websites, usually with similar or related content, and all of which link back to a single "money" site that the owner is attempting to rank higher in search engine results. PBNs typically consist of expired or deleted domains that still have existing backlinks pointing to them.
PBNs are considered a form of black hat SEO, as they violate search engine guidelines by attempting to manipulate search results through link schemes. If a PBN is detected by search engines, the owner's website can be penalized or even banned from search results altogether.