Reach
Reach is the number of unique people who saw a piece of content at least once. It is the honest denominator for claims like 'we reached X people', and it is only visible in the creator's own analytics, never publicly. A typical post reaches a fraction of the follower count, which is why follower totals overstate what a placement buys.
Worked example
A post shown to 8,200 unique accounts has a reach of 8,200 — even if it was displayed 19,000 times.
Related: Impressions · Engagement rate
Full context in the guides, or measure it yourself with the free tools.