Reach
Number of unique users who saw your post at least once.
Reach counts unique people, not events. If the same user sees your post 5 times, that's still 1 reach but 5 impressions. Reach is the truer measure of audience size for a given piece of content. Organic reach has declined sharply across all platforms since 2020 (Facebook organic reach is now under 5% of page followers; Instagram around 10-15%). Paid promotion is the only reliable way to extend reach beyond your existing audience plus algorithmic recommendations.
Related terms
Total number of times your post was shown, including repeat views.
Percentage of followers who interact with a post (likes + comments + shares + saves divided by reach or followers).
The ranking system each platform uses to decide whose content appears in whose feed.