Impressions
The total number of times a post, story or ad was shown, counting repeat views.
Impressions count every showing. One person scrolling past a post three times is three impressions and one person reached. Impressions are always equal to or higher than reach.
Impressions are the base for CPM and EMV, which is why the definition matters: some platforms count a view as soon as a post appears on screen, others only after a short time. When you total impressions across platforms, you are adding slightly different things together, and that's fine as long as the report says so.