Glossary
What is Impression?
Also called: ad impression
An impression is one ad served once on a page or in an app. Most display ads are priced per 1,000 impressions (CPM).
हिंदी में पढ़ेंAn impression is counted when an ad request comes back with an ad and the ad serves in the slot. It is the basic unit of display advertising: buyers usually pay a CPM, a price per 1,000 impressions, and fill rate, eCPM and viewability are all built on impression counts.
An impression is not proof that anyone saw the ad. A viewable impression is a stricter count: at least 50% of the ad’s pixels on screen for one continuous second, or two seconds for video. So viewable impressions are always a subset of impressions.
When an impression counts is changing. Today Google Ad Manager and AdSense count most web display banners when the ad starts downloading. From 17 February 2027 they move to begin-to-render counting, so an impression counts only once the ad starts to display. Google says total display impressions may fall.
Example
A page sends 1,000 ad requests and 900 return an ad: that is 900 impressions. At a $2.00 CPM they earn $1.80 (900 × $2.00 ÷ 1,000).
Why it matters to publishers
Impressions are what most buyers pay for, so a fall in impressions can mean a fall in revenue. Knowing how they are counted helps you read reports correctly.