Impression

An impression means the ad was delivered. That is a lower bar than it sounds, and the gap between it and “someone looked at this” is where a great deal of budget goes quietly.

Three separate things, one word.

Served — the ad was sent in response to a request. It may have rendered below the fold, in a background tab, or in a frame nobody scrolled to.

Viewable — some proportion of the ad’s pixels occupied the screen for some period of time. The exact thresholds differ by format and by standards body, which is worth checking rather than assuming, and “viewable” is still a statement about geometry, not about attention.

Seen — a person actually registered it. Nothing in ad delivery measures this. It is not a metric you can buy reporting on.

Why the distinction costs money. Impressions are the denominator of the cost metrics, so cheap impressions make every efficiency figure look better while buying less of what you wanted. And impressions are the trigger for view-through conversions — buy more of them and the credited conversions rise mechanically, whether or not anyone engaged.

The practical question to ask about any impression-based number is which of the three it counted, and whether the answer came from the party selling the inventory. Where it matters, the way out is not a better impression metric but a holdout test: stop serving them to a comparable group and see whether anything changes.