Impression

An impression is counted each time an ad is served and rendered to a user, regardless of whether it was seen, read, or interacted with. It is the base unit that most other advertising metrics are built from: cost per mille prices a thousand of them, reach and frequency describes how impressions are distributed across an audience, and click-through rate divides clicks by impressions.

A raw served impression is not the same as a viewable one. Ads placed below the fold, loaded but never scrolled into view, or served to a browser tab in the background can all count as served impressions without a real chance of being seen, which is why ad viewability standards exist as a stricter, verified subset of impression counts. Impressions also differ from reach: one user can generate many impressions of the same ad, which is what frequency capping is designed to control.

Impression counts matter for planning and buying media at scale, but they are also the metric most inflated by invalid traffic, since bots can generate served impressions just as easily as real users. Reporting on impressions without checking viewability or traffic quality is a common way campaign performance gets overstated.