Measurement
Viewable impression
A viewable impression is an ad impression that meets a viewability standard, by default the MRC's: 50% of the ad's pixels on screen for one continuous second, or two seconds for video.
An impression is counted when an ad is served. A viewable impression is the subset that actually appeared on screen long enough to be seen, according to agreed rules. It turns a technical event into something closer to a real opportunity to see.
The common definition comes from the MRC and IAB Viewable Ad Impression Measurement Guidelines: 50% of pixels in view for at least one continuous second for standard display, 30% for one second for large-format display, and 50% for two continuous seconds for video. Measurement must also filter invalid traffic: a bot "viewing" an ad does not create a valid viewable impression.
Viewable impressions are the basis for vCPM pricing and many campaign KPIs. They can still vary between vendors because of measurement methods, which is one reason for discrepancies. Remember that viewable is not the same as viewed: Attention metrics try to go further by estimating whether anyone looked.
Think of it like this
It is like counting a flyer as "delivered" only if it landed face-up on the doormat, not stuck in the letterbox flap.
An example
A video ad in Japan plays in a feed; it is on screen at 80% for 1.5 seconds before the user scrolls away. It counts as an impression but not a viewable impression under the two-second video rule.
Related terms
Viewability
Viewability measures whether an ad actually had a chance to be seen on screen; the MRC standard is 50% of pixels in view for one second (display) or two seconds (video).
Impression
An impression is one instance of an ad being served and counted as displayed to a user; it is the basic unit that most online advertising is bought and sold by.
MRC (Media Rating Council)
The MRC (Media Rating Council) is a US non-profit industry body that sets measurement standards and audits and accredits media measurement services, including viewability and invalid-traffic filtering.
Attention metrics
Attention metrics estimate how much people actually noticed or engaged with an ad, using signals like time in view, screen share, interaction and eye-tracking panels, beyond simple viewability.
CPM (cost per mille)
CPM (cost per mille) is the price an advertiser pays for one thousand ad impressions; "mille" is Latin for thousand.