What is an ad impression?
An impression is when an ad is fetched from its source, and is countable. Whether the ad is clicked is not taken into account. Each time an ad is fetched, it is counted as one impression.
Source: Wikipedia
An ad impression (also known as a view-through) is when a user sees an advertisement. In practice, an impression occurs any time a user opens an app or website and an ad is visible. Each time an ad is served and viewed, it is counted as an impression. Impressions are not to be confused with engagements.
Ad impressions are essential in mobile advertising. Mainly measured when working with a cost per mille (CPM) model—where advertisers pay for every thousand impressions served—advertisers use impressions to evaluate ad reach across channels and optimize campaigns, while publishers use them to calculate effective cost per mille (eCPM) and maximize revenue. Accurate impression measurement is critical for calculating metrics like click-through rate (CTR).
Additionally, impression data can help mobile marketers drive improved user acquisition (UA). More ad views mean greater exposure to potential users. For example, if you’re promoting a mobile game and running in-app banner ads targeted at casual gamers, measuring impressions lets you see how often your ad is displayed within other gaming apps. This helps you assess whether the ads are being shown to the right audience and allows you to adjust pricing, placements, or creatives to boost visibility and installs.
App store optimization (ASO) also benefits from impression data. If ad views are high but downloads are low, it signals a need for improvements in metadata like app store title or description. Marketers can optimize these elements to boost ad visibility and acquire more users.
Types of ad impressions
While ‘impressions’ broadly refers to ad views, ad impressions do vary somewhat in how they measure visibility and interaction with ads. Here’s a breakdown of the key types:
Served impressions: These occur when an ad is delivered to a web page, regardless of whether the user sees it. This is the most common way to measure impressions and serves as a baseline metric for ad reach. However, served impressions may include ads that are not visible, potentially overstating how many users actually see the ad.
Viewable impressions: These measure the number of times the target audience actually sees an ad. According to the Media Rating Council and Interactive Advertising Bureau, an ad is considered viewable if at least 50% of it is visible on an active browser tab for at least one second. This type of impression provides more accurate data on ad visibility but can be harder to measure, as not all ad servers count viewable impressions.
Verified impressions: These confirm whether an ad was shown to a real user, reducing the risk of fraud. For advertisers, verified impressions ensure that ads are not being delivered to bots or non-human traffic.
Engagement impressions measure how often users interact with an ad, including actions like expanding the ad or clicking on elements within it. Engagement impressions help advertisers assess which ads generate the most user interaction and interest. This can also refer to video impressions where the content was viewed for a specific period of time (like 10 seconds) implying the impression was an ‘engaged’ one.