a Greek: a person of the Greek-speaking world, and so, from a Jewish standpoint, any non-Jew
The word literally names a Greek, someone who belongs to the Greek language and culture that spread across the ancient Mediterranean. In that plain sense it stands opposite the foreigner who does not speak Greek, as when Paul says he is a debtor both to Greeks and to non-Greeks (Romans 1:14).
But Jews divided the world in two: themselves and everyone else. Because the Greek world was the non-Jewish world they knew best, "Greek" came to stand for any non-Jew at all. So when Paul writes "to the Jew first and also to the Greek," he does not mean only people from Greece; he means the whole of humanity outside Israel.
The two uses shade into each other and are usually easy to tell apart by context. In the book of Acts the "Greeks" are often the actual Greek residents of the cities Paul visits, alongside the local Jews. In Paul's letters "Greek" is usually a shorthand for "Gentile." One word carries both.
This is a different word from "Hellenist," which meant a Greek-speaking Jew. A Greek here is a Gentile, not a Jew who happened to speak Greek.