In the simplest of terms, a viewable refresh is an ad unit that only refreshes when in view to the user.  The industry standard definition of “viewability” for digital ads, defined by the MRC (Media Ratings Council) standard for “viewability”, states that 50% of the pixels of the ad must be in view for 1 second (display ads) or 2 seconds (video ads). Refreshing ads when they are not in view inflates impressions and negatively impacts performance and campaign profitability.