Viewability is a metric or a measurement technique that is used in digital advertising to track and evaluate the visibility or the exposure of an ad to the user. Viewability is typically used for display or video ads that are served through an ad server or a tracking pixel, and that are eligible for billing or attribution, depending on the standards or the guidelines of the industry, the platform, or the publisher. Viewability is calculated as the ratio or the percentage of ad impressions that were actually viewed by the user, within a certain period of time or a certain distance from the ad, divided by the total number of ad impressions. Viewability is an optional and complementary metric to reach or frequency, and it is used to assess the quality or the value of an ad placement, the inventory, or the targeting, beyond the quantity or the performance of the ad.