A zero-day (also known as a 0-day) is a vulnerability in software or hardware that is typically unknown to the vendor and for which no patch or other fix is available. The vendor has zero days to prepare a patch as the vulnerability has already been described or exploited.

From wiki

My understanding, zero day means when the exploit was discovered it was already being used in the wild. This is different from an exploit discovered by a bounty program or by security researchers.