Impression
A single view of an ad by a user.
An impression is counted every time an ad is rendered on a user's screen — even briefly. One user scrolling past the same ad twice in a day is two impressions. Impressions are the denominator of CTR, CPM and frequency.
Not all impressions are equal: a 0.5-second view in a fast scroll is the same as a 5-second view in terms of impression count, but obviously very different in value. That is why platforms now report 'ThruPlay' (15s+ video view) and 'on-screen' separately. Prefer those metrics for video campaigns.