An impression is counted for each ad request where at least one ad has begun to download to the user’s device. It's the number of ad units (for content ads) or search queries (for search ads) that loaded ads.
A note about impression counting
The Interactive Advertising Bureau (IAB) and the Media Rating Council (MRC) in partnership with other industry bodies, periodically review and update industry standards for impression measurement. They recommend guidelines to standardise how impressions are counted across formats and platforms. Over time the standards for counting an ad impression on the web have evolved: from counting an impression when the ad is served, to counting an impression when the ad starts to download on the user’s device (i.e. the ad begins to load on a page).
The definitions are summarised below:
- Served impression: An impression is counted when an ad is sent (or 'served') to a publisher by the ad server. The ad creative may or may not be downloaded to the user’s device, rendered or viewed by the user.
- Downloaded impression: An impression is counted only after the ad has begun to download to the user's device.
The AdSense 'Impressions' metric uses the downloaded definition for all impressions. Note that the complementary ActiveView viewable metric is used to determine if users actually viewed the impressions.