What did “Kingdom of Greece” refer to in Daniel 8:21?

I was reading Daniel 8:15-27, where it says:

…. the ram you saw that had two horns represents the kingdoms of Media and Persia. The goat represents the kingdom of Greece, and the prominent horn between his eyes is the first king….
—Daniel 8:20-21 (GNT)

Can anybody explain what is meant by “kingdom of Greece”? Which countries/region/area does it point to now or back in the day?