The number of unique accounts that saw a piece of content at least once — distinct from impressions, which counts repeated views.
Reach counts unique viewers. If 1000 people saw a post and 200 of them saw it twice, reach is 1000 and impressions are 1200. Reach is the more honest measure of "how many distinct humans encountered this content."
Reach is driven by three things: follower count (who sees it directly), distribution to non-followers (the explore page, recommendations, hashtag pages), and shares (which expose it to followers' followers). The third lever is the most leveraged — a single viral share can multiply reach 100x.
Reach degrades fast on most platforms. Instagram's organic reach for a typical account hovers around 10–20% of follower count; LinkedIn's around 5–10%. Paid promotion bypasses the cap. The way to compound reach organically is consistent saves/shares/comments, which trigger the algorithm to push the content beyond followers.