Impressions
An impression is one delivery of an ad to a screen, counted whether or not anyone noticed it. Impressions measure volume of delivery, not people reached - one person seeing an ad ten times is ten impressions and a reach of one.
In practice
Impressions are the raw currency of most digital buying (CPM pricing) and the input to reach and frequency maths, but they are a poor success measure on their own.
Pair impressions with viewability and frequency to understand whether delivery was actually seen and how it was distributed across people.