What is Impressions?
Impressions are the total number of times a piece of content was displayed, counting every view including multiple views by the same user.
Unlike reach, impressions are not deduplicated. If one person sees the same post three times in their feed, that counts as 3 impressions. High impressions relative to reach indicates the algorithm is serving your content repeatedly to the same users.
CPM (cost per mille) is calculated against impressions, making impression counts central to paid media and influencer deal pricing. Brands paying on a CPM basis care about total impressions delivered, not just unique viewers.
Creators reporting campaign performance should always clarify whether they are citing reach or impressions, as the two are often confused and the difference can be significant in post-campaign reporting.