Ad viewability refers to the measurement of whether an online advertisement is actually seen by users. It is determined by factors such as the ad's placement on a webpage and how long it remains in view. For an ad to be considered "viewable," at least 50% of its pixels must be visible on the screen for a minimum of one second for display ads, and two seconds for video ads. This concept is crucial for advertisers and marketers, as it helps them assess the effectiveness of their campaigns. High viewability rates can lead to better engagement and return on investment, while low rates may indicate issues with ad placement or user attention.