What is viewability in the context of online advertising?

Viewability refers to the probability that an ad will be seen by a user. In online advertising, this concept is crucial because it helps advertisers assess the effectiveness of their ad placements. An ad is considered "viewable" when it meets certain criteria, such as being displayed on the screen for a minimum amount of time and at least a portion of the ad being visible to the user.

Understanding viewability is vital for optimizing ad spend and ensuring that advertisers are getting value from their campaigns. Advertisers focus on viewability to maximize the chances that their messages reach the intended audience effectively, thus increasing the likelihood of engagement and conversions.

The other options, while related to advertising, do not appropriately define viewability. Some pertain to different metrics, such as engagement rates or reach, rather than the specific measurement of whether an ad is likely to be seen.