Visibility concentration measures how much of a category's AI-answer visibility is held by a few dominant brands. It is usually expressed as the share of total category visibility owned by the top three brands: a high share means answers in that vertical are dominated by a handful of names, and a low share means the field is contestable.
Concentration is a property of the category, not of AI search overall. Semrush's 2026 AI Visibility Index found the top three brands held roughly 82.9% of News & Media visibility and 76.9% of Consumer Electronics, but only about 41.4% of Finance and 42.2% of Industrial (126M US prompts, January–April 2026, single-vendor and directional). So the same engines behave close to winner-take-all in one vertical and leave real room to climb in another.
It is distinct from Share of Voice, which measures your brand's presence, whereas visibility concentration measures the market structure around you. It also differs from a citation fingerprint (which page types get cited) and from domain concentration (which domains engines lean on generally). Reading your category's concentration first tells you whether to compete for the crowded head directly or to go around it with long-tail depth and earned placement.