Viewability
Whether an ad was actually rendered in a position where a person could have seen it, usually defined as 50% of pixels for one second.
The standard definition is deliberately weak: half the ad, for a single second, is a low bar and still a large share of display inventory fails it. That is the point — it separates "served" from "had any chance at all".
Viewability applies to display formats. It does not apply to a newsletter sponsorship, a podcast read or a sponsored post, where the ad is inside the content rather than beside it, and a buyer asking for a viewability guarantee on those is asking for a number nobody can produce.
Terms it depends on
- Impression
- One instance of an ad being served — not necessarily seen, and not necessarily by a person.
- CPM
- The price of one thousand ad impressions — the standard unit for buying reach rather than outcomes.