Impression

An impression is the smallest billable unit in advertising: one opportunity for one person to see one ad. In web display, this is one creative render in a viewable region. In CTV, it's one full playout to one device. In DOOH, it is one playout multiplied by the audience-on-screen during that playout.

The multiplication step is what makes DOOH measurement different. A single 15-second playout on a bar TV with 8 viewers is 8 impressions, not 1. The audience-on-screen figure comes from one of two sources: a modeled audience derived from venue foot-traffic (the Geopath/DPAA standard, dominant in non-sensor inventory) or a sensor-counted audience derived from real-time computer vision (the Trillboards approach, growing in modern inventory).

The IAB's distinction between an "impression" and a completed impression matters here. An impression is the opportunity to be seen; a completed impression is the playout having finished. DOOH bills on completed impressions because there is no user-controlled pause, once the slot starts, it runs to the end barring a screen-side failure. Web display rarely makes this distinction because the equivalent of "completion" is "still in the viewport at impression time".

Impressions are denominated in thousands when priced, see CPM.