Official permission (stamp/loose document) for a person to leave the country they are visiting or living in.
This requirement applies generally to expats, business people or people traveling on working visas to ensure taxes on income are fully paid before leaving the country.
Apparently, some countries do impose this requirement on their own people e.g Iran (at least up to year 2000). Cuba abolished it in 2013. etc.
Uzbekistan abolished theirs at the beginning of 2019. As far as I know, North Korea is the only country that still has it for their own citizens. China has a more subtle system that doesn't have exit visas, but requires permission from your government employer before issuing a passport.