Impression

An impression is counted each time an ad is served and rendered on a screen, regardless of whether anyone noticed or clicked it.

An impression is the loosest unit of measurement in advertising — it says an ad loaded, not that a human saw it. Viewability standards (like the IAB's "50% of pixels in view for at least one second") exist specifically because raw impressions include ads that loaded off-screen, in a background tab, or for a fraction of a second before a user scrolled past.

Impressions are the denominator for both CPM and CTR, which is why comparing raw impression counts across platforms with different viewability standards can be misleading.