Share of voice
The percentage of AI answers about your category in which your brand appears, measured against competitors.
Share of voice is comparative, which is what makes it useful. A raw mention count only tells you that you appeared; share of voice tells you whether you are winning or losing the answer against specific rivals.
There is no single formula. The two common methods are mention-count share, your mentions divided by the total across you and your competitors, and presence share, the portion of tracked prompts where your brand appears at all.
If an engine names a competitor in eight of ten category questions and you in three, the problem is concrete enough to prioritize, which is why it is usually the most actionable number in AI search.