A technique that allows multiple demand sources to bid on ad inventory simultaneously before the ad server makes a final decision on which ad to display. This increases revenue for publishers by enabling them to sell their inventory to the highest bidder. In contrast, traditional programmatic advertising uses the waterfall system, a sequential process where ad inventory is sent to different demand sources in a predetermined order. However, this system has a flaw that the highest bid can lose to a lower bid from a later demand source in the order.