Advertising
Impression
One instance of an ad being served, which is not the same as an ad being seen.
What makes it wrong.
The mechanism, not the warning.
Mixing served and viewable impressions in the same calculation silently changes every rate built on them. A CTR computed against viewable impressions is not comparable with one computed against served impressions, and neither is labelled in most exports.
In detail.
An impression is recorded when the ad is delivered. Whether it entered the viewport, for how long, and whether a person was looking are separate questions that separate metrics answer.
Platforms that also report a viewable or qualified impression are reporting a subset, sometimes a much smaller one.