The measure of an ad being viewed by whoever is watching the content in which the ad is delivered. This metric, or some form of it, is often used as the basis upon which digital advertising is bought and sold. In practice, an impression for a video ad typically occurs if 50% of the pixels of the ad are visible on the screen for at least two continuous seconds. This is a key part , but not the only part, of the standard developed by the Media Ratings Council (MRC) as a guideline to help define when an impression has been met.