Definition of Viewable impression:
A measure of the performance or effectiveness of online advertising when a site appears weak. Visual impressions were developed by the online advertising industry in the mid-1990s to increase the measurement of ad success on Internet servers. Such metrics cannot provide or determine information about events. Whether or not the content displayed in the online ad is actually visible to the public. Meanwhile, viewers were surfing. Realo preserves the visible image after loading and processing the content of this ad, and most of the content appears in the window for at least a second. The ability to click on the destination site is enabled as soon as the viewer impresses.
Meaning of Viewable impression & Viewable impression Definition