The industry standard for viewability was developed under the leadership of the Media Rating Council (MRC).
Display and App ads are considered viewable when at least 50% of the ad is visible for at least 1 second.
Video ads are considered viewable when at least 50% of the ad is visible while the video is playing for at least 2 consecutive seconds.
For larger ad units (like Skins), 30% of pixels in view for 1 second constitutes a Viewable ad.
According to IAB, "custom ad units and important elements of sponsorships are not consistently measurable today. The measurement standard and the technology are still evolving."