In AI visibility, share of voice is the percentage of relevant AI answers that mention a brand, across a defined set of prompts, models, and time.
Classic share of voice measured advertising weight; the AI version measures answer weight. If ChatGPT is asked 100 buying questions in a category and names a brand in 40 of the answers, that brand holds a 40% share of voice on ChatGPT for that period.
Share of voice and rank answer different questions. Rank tells you where you stand when you are named; share of voice tells you how often you are named at all. A brand can rank #2 with a thin share of voice — recommended prominently, but rarely.