Impressions that do not meet certain ad serving quality or completeness criteria, or otherwise do not represent legitimate ad impressions that should be included in impression counts. Among the reasons why an ad impression may be deemed invalid is it is a result of non-human traffic (spiders, bots, etc.), or activity designed to produce fraudulent impressions.
Impressions that result from an intentionally deceptive practice designed to manipulate legitimate ad serving or measurement processes or to create a fictitious activity that leads to inflated counts. All fraudulent impressions are invalid (but not all invalid impressions are fraudulent), and impressions known to be fraudulent can never be viewable impressions. Some viewable impression measurers apply additional techniques to identify suspected fraudulent impressions subsequent to making a viewability determination about an ad and account for these in their reported impression counts.
Source: TrafficGuard Glossary