← Glossary · Measurement & Metrics Measurement & Metrics Definition varies 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. Also known as: ad impression Source: IAB DOOH Metrics Glossary 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 Multiplier A factor applied to each ad play to estimate how many people were in viewing range, derived from audience data such as foot traffic and dwell time. Methodologies differ between vendors, so multipliers are not always comparable. CPM (Cost per Mille) The price for one thousand impressions — the standard currency for buying DOOH. Impressions are the basis of any CPM-based transaction. See also Research Beauty DOOH market sizing: a bottom-up modelThe impression multiplier, explained