Delivery of less impressions, visitors, or conversions than contracted for a specified period of time.
Underdelivery can occur for a variety of reasons. A site or network may experience an unexpected drop in traffic. Low CPM campaigns may be bumped for high CPM campaigns. Pay-for-performance may be bumped for any CPM campaigns, plus there is the added risk that the creative units fail to generate the anticipated level of response.
It is generally advisable to plan solutions for underdelivery before an advertising buy is completed, rather than trying to haggle on a settlement after the fact.
see also: Online Advertising Marketing Tools