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Real-time bidding (RTB)
Real-time bidding is the auction that sells a single ad impression to the highest bidder while the page is still loading.
When a page with ad space loads, the publisher's side sends a bid request describing the impression and the visitor to many buyers at once. Each decides automatically whether to bid, and the winner's ad is served — all in a fraction of a second.
It is the engine of programmatic display advertising. It is efficient at scale, but the price and placement of each individual ad are invisible to a small advertiser.