Surely all 0days are known about by someone for some period of time before they actually get used? Or do you think people discover these exploits and then use t

Surely all 0days are known about by someone for some period of time before they actually get used? Or do you think people discover these exploits and then use them on the same day? My understanding was that 0day just meant that it was unpatched, not "publicly" known and being used for the first time "in the wild".