What is an impression in advertising?
An impression is one instance of an ad being served and displayed on a screen, whether or not anyone clicks or even notices it. It is the basic counting unit of digital advertising: campaigns are planned, priced and reported in impressions, usually bought per thousand (CPM). Industry rules define when an impression counts, typically when the ad begins to render.
An impression differs from a viewable impression, which requires the ad to actually be on screen for a minimum time, and from a click or conversion. Counting guidelines from the IAB and MRC specify that an impression counts when the ad starts to render on the device, not merely when it is requested.
Not every impression is human: Invalid traffic filters remove impressions from bots, and sophisticated fraud tries to fake them. Measuring reach and frequency capping turns raw impressions into how many people saw the ad and how often.