← Glossary · Measurement & Metrics Measurement & Metrics Definition varies Viewable Impression An impression that met a viewability condition (the ad was actually on-screen and seeable). The relationship to VAC/audience impression is debated — some bodies treat viewable impressions as a bottom-up counterpart to VAC’s top-down approach. Also known as: viewable impression Source: MRC OOH Measurement Standards Related terms Audience Impression The canonical DOOH measurement unit: the total number of people with an opportunity to see (traffic), adjusted for notice and dwell time and calibrated to the spot length. The same concept the industry also calls Visibility Adjusted Contact (VAC). Opportunity to See The probability of being exposed to a medium’s content or advertising. OTS does not require actual exposure to all content — it counts the chance to see, not confirmed viewing. Impression A counted exposure of an ad to a person. In DOOH it is the foundational unit of currency, but the exact definition varies by standard — from a raw play, to an opportunity-to-see, to a viewability-qualified audience impression. See also Research Attention as the new currencyMeasurement maturity: DOOH vs CTV vs display