a member of the people of Judah: a Judean, and later a Jew
This word began as a simple label for where a person came from: someone who belonged to Judah, the southern kingdom and its territory. In the earliest passages it works like any place-based name, the way we might call someone a Texan or a Roman. A Judean was a citizen of Judah, set apart from the people of Israel to the north or from the surrounding nations.
After Judah fell and its people were scattered, the same word slowly took on a fuller meaning. A person could now be one of these people while living far from the land of Judah, in Babylon or Persia or anywhere across the empire. What marked them was no longer an address but a shared ancestry, a shared God, and a shared way of life. This is the sense English Bibles usually translate as Jew.
The two shades are not two different words but one word stretching across a turning point in the nation's history. The book of Esther shows the later sense at full strength: the people are spread through one hundred twenty-seven provinces, their identity is something others can even choose to join, and the threat against them is aimed at the whole people rather than a country. The historical books and Jeremiah show the earlier, territory-based sense.
Because the shift is gradual, some verses sit comfortably in either sense, and translators differ on where the line falls. The point to hold onto is that the word names belonging to a people, whether that belonging is pictured chiefly as a homeland or chiefly as a heritage.