Ad viewability relates to the amount of pixels within an ad that can be seen in a given set of time. This in turn relates to the amount of times the ad is viewed and the potential uptake on the ad. There is no set definition for when an ad becomes viewable but it is agreed to be over 50% in all cases with some considering an ad to only be visible when 100% of the pixels have loaded.
In order to gain the highest level of ad viewability ads will use graphics that can be easily viewed on any type of device and using any speed of internet service. In this way the ad will be optimized to be readable to the largest audience. Metrics will measure how many seconds a visitor to an ad stays on it, where they go after they leave and whether the ad leads to a purchase. If the ad is left quickly it would be tweaked to get viewers to consider the offer longer and potentially make a purchase.