Viewable impression

A viewable impression is a display ad with at least half its pixels on screen for one continuous second.

The threshold is the Media Rating Council standard: 50% of pixels for one second for display ads, and for two continuous seconds for video. Ad servers report viewable impressions separately from served ones.

Dividing a CPM by the viewability rate gives a vCPM, the price per thousand ads that could actually have been seen.