Share of voice
Share of voice is how much of the conversation is yours: of all the brand counted in scope, the proportion that were you.
It is competitive, which makes it different from visibility. Visibility is how often you appear and nobody else can change it. Share of voice falls when rivals get mentioned more, even if you are mentioned exactly as often as before.
Only you track are counted, so the shares of your tracked brands add up to 100%. Dividing the conversation among every name ever spotted would leave everyone near zero.