Impression
One instance of an ad being served to a user.
What is impression?
An impression is the counted event of an ad being delivered to a page, app, screen or stream. It is the fundamental unit of digital advertising for pricing, delivery and reporting, and underpins metrics such as CPM, click-through rate and fill rate. Industry measurement guidelines generally count an impression when the ad begins to render or download on the device, rather than when the ad server merely decides to send it. Because delivery does not guarantee a person saw the ad, buyers also track viewable impressions, and in video they track starts and completions.
How impression works
An impression is counted when the ad is delivered. Whether it was actually seen is measured separately as viewability.
- A page, app or player sends an ad request for an available slot.
- The ad server or auction selects an ad and returns the creative.
- The ad begins to render on the device and fires an impression tracker.
- The ad server and any third-party trackers log the impression for billing and reporting.
Why it matters
It is the basic unit in which most media is bought, sold and counted.
For advertisers
Impressions measure reach and delivery against plan, and form the basis of reach-oriented pricing and frequency control.
For publishers
Impressions are the inventory publishers sell, so counting methods directly affect revenue and reconciliation with buyers.
Practical example
A page with three ad slots generates three impressions per page view.
Key considerations
- Counts often differ between the publisher’s and buyer’s systems, so discrepancy thresholds are usually agreed in advance.
- Invalid traffic filtering removes impressions generated by bots or fraud.
- Served, rendered and viewable impressions are different counts and should not be compared directly.
More Measurement & Optimization terms
Frequently asked questions
What is the difference between an impression and a view?
An impression is counted when an ad is delivered; a viewable impression additionally meets a standard for being on screen.
Why do impression counts differ between platforms?
Timing of tracking calls, filtering rules and network failures can cause small discrepancies between systems.
How are impressions counted in DOOH?
Often with impression multipliers that estimate how many people could see each ad play.