Metrics
Viewability
Viewability measures whether an ad had the opportunity to be seen, commonly using the MRC standard of at least 50% of pixels in view for one second (display) or two seconds (video). It filters delivery quality; it does not confirm anyone actually looked.
In practice
Viewability is a hygiene metric: buying unviewable impressions is buying nothing. But 100% viewable and ignored is still ignored - attention metrics attempt the next step.
Set viewability expectations by format and placement in the plan, and verify with measurement rather than platform self-reporting where stakes are high.