Reach

Reach is the number of unique people who saw a post. If one person sees it four times, that is a reach of one.

Reach versus impressions

Impressions count every time a post was shown, including repeats. Impressions are always the higher number, and the gap between the two tells you how often the same people are seeing you.

Neither is the same as views, which platforms define differently and sometimes count after a second or two of watch time.

Why the distinction matters when pricing

A brand paying for reach is buying how many people heard the message. A brand paying for impressions is buying how many times it appeared. Quoting one while they are budgeting for the other is how a fee ends up looking wrong to both sides.

The awkward part

Platforms do not agree on these definitions and change them without much warning. A number labelled "reach" in one app is not necessarily measuring the same thing as the identically labelled number in another.

The practical answer is to compare a number against itself over time rather than against a different platform's version of it, and to say which platform any figure came from whenever you quote it.