Reach
In a Nutshell
Reach is the number of unique people who saw a piece of content. In theory: a measure of visibility. In practice: the number where 'how many saw it' gets confused with 'how many cared.'
What It Really Means
Reach shows how many people a piece of content got to. Big numbers look impressive and read well in reports.
But reach alone says nothing. It is possible to reach a million people and move no one. Being seen and being remembered are not the same.
The years-long decline of organic reach is the result of platforms' tendency to make content paid. Today, wide reach is often bought with budget.
Reach is the beginning of the story, not the end. The real question is not 'how many did you reach,' but 'what did you leave with those you reached.'