Pay for placement
Pay for placement, or P4P, is an Internet advertising model in which advertisements appear along with relevant search results from a Web search engine
Web search engine
A web search engine is designed to search for information on the World Wide Web and FTP servers. The search results are generally presented in a list of results often referred to as SERPS, or "search engine results pages". The information may consist of web pages, images, information and other...

. Under this model, advertisers bid for the right to present an advertisement with specific search terms (i.e., keywords
Keyword (Internet search)
An index term, subject term, subject heading, or descriptor, in information retrieval, is a term that captures the essence of the topic of a document. Index terms make up a controlled vocabulary for use in bibliographic records. They are an integral part of bibliographic control, which is the...

) in an open auction
An auction is a process of buying and selling goods or services by offering them up for bid, taking bids, and then selling the item to the highest bidder...

. When one of these keywords is entered into the search engine, the results of the auction on that keyword are presented, with higher ranking bids appearing more prominently on the page.

When P4P was first introduced, controversy arose because seventy percent of Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...

users were unaware that search results could be skewed as a result of such agreements, which in some cases led to legal action.
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