The pre-header-bidding method of selling unsold inventory: the ad server offers the impression to demand sources one at a time in priority order, each at a fixed price, until one accepts. Waterfalls undervalue impressions because higher-ranked partners never see competing bids. Most publishers replaced them with Header Bidding, though waterfalls persist in some in-app mediation.
Related terms
Used in The Signal
Cite this entry:
Agency Echelon. "Waterfall." Agency Echelon Reference, reviewed September 2026. https://agencyechelon.com/reference/glossary/waterfall/