A link which has been purchased, usually with the intent of improving a page’s ranking in search results.
While the practice of buying links is not intrinsically bad (web advertising is purchasing links), Google guidelines prohibit selling links that pass PageRank (affect Google’s PageRank algorithm). Other search engines have similar policies. Sites which buy or sell such links can be penalized or removed from search engine rankings.
Note: The FCC also requires persons being paid to promote a company’s product to disclose their relationship to the company.