An impression is counted every time an ad appears on a screen. It does not matter whether anyone looked. It does not matter whether anyone clicked. The ad rendered, the server logged it, and the counter ticked up by one.

This makes impressions a crude measure, but a necessary one. Almost every other digital metric builds on them. Click-through rate divides clicks by impressions. Cost per mille prices a thousand of them. Without impressions, none of that math exists.

Where impressions accumulate

• Display banners on websites and apps
• Sponsored posts in social feeds
• Video pre-roll before streaming content
• Search results pages

The number can mislead. A single user refreshing a page ten times generates ten impressions. Advertisers who chase raw volume without checking unique reach often pay for the same eyeballs again and again.