Share of voice reframes a raw mention or citation count as a percentage of the total activity across a query set, split between you and named competitors. A count of 60 mentions means little on its own; a 12% share against a competitor holding 55% is immediately legible. It applies to traditional SERP visibility and, increasingly, to AI citation tracking, where it answers "are we winning" rather than just "are we visible."
See GEO Competitor Analysis: See Who's Winning in AI Search for the full picture.