The distinction drives different decisions. Reach answers "how many people did this touch?" — the metric for awareness and for judging whether content escaped your existing audience. Impressions answer "how much total exposure happened?" — the currency of ad pricing (CPMs are paid per impression, not per person).
Platform vocabulary keeps shifting: Instagram retired impressions as a headline metric in favor of "views" (counted per play, including replays) with reach still reported as unique accounts, and X reports impressions per tweet. An impressions-to-reach ratio well above 1 means people return or content re-surfaces repeatedly — often a good sign for resonance, or for Stories, a sign of sequence re-watching. When reporting to brands, defining which metric a number refers to avoids the most common analytics misunderstanding in sponsorship recaps.
