Visibility in online ads is a measure of whether an ad has been seen and varies between ad servers and analytics platforms.
Visibility is a metric that focuses on ad impressions that users actually see. For example, an impression isn't considered viewable if the user doesn't scroll down the page far enough to see the ad. MRC standards, introduced in 2014, require 50% of display ads to remain on screen for one second (two seconds for video ads) before billing the advertiser.
Visibility is all impressions your users have seen.