Viewable impression is a term used for online advertising metrics. It refers to an ad delivered onto a web page in a place that could be seen by the user. The push for a standard metric to be used to measure this type of ad impression was borne out of frustration among advertisers paying for ads that were served to parts of pages not always seen by users, such as the bottom of a page.
The Media Ratings Council's standard for a viewable display impression is a minimum of 50% of pixels in view for a minimum of one second. The decision has sparked criticism.
Kyle Langley, chief research officer at BFG Communications
A viewable impression is a metric of online advertising representing when an ad can actually be seen -- or viewed -- by a person on a web browser. Under the old system of "online impressions," there were limits on the ability to measure if ad content was actually visible to a viewer. Now, those measures can be precisely targeted to specific areas on the browser's landscape. Viewable impressions are a huge improvement over online impressions and give marketers a more precise measure to know if ads are actually viewable, how often the ads are seen, how often they are clicked and extrapolate that to ROI, media spend and marketing effectiveness.
Knowing that an end user can actually see an ad vs. only that the ad is placed, gives advertisers a clearer picture of the power an ad has to attract clicks (known as click-through, which is also measured).
Proving the need for a new measurement system, ComScore released a report that found 31% of online ads were previously not viewable by the end user, while RealVu and C3 have claimed that up to 80% of ads are never even within eyeshot. Therefore, having a metric like viewable impressions provides true ad viewability and has the potential to be far more accurate and effective.