Impression fraud is the fraudulent practice by which ads are delivered and counted as served without having any chance to be seen by humans. Impression fraud is theoritically distinct from classic ad viewability issues.
Impression fraud is especially met on some adexchanges and second tier networks. The phenomenon is difficult to evaluate. Some studies estimate that up to a third of all ad impressions are fraudulent, but most often they are created and published by ad verification solutions.
See also click fraud.