a people group bound together as a body: a nation; in the plural and from a Jewish vantage point, the other nations, that is, the Gentiles
At its root this word names a body of people who belong together, a nation or people defined by shared descent, land, and custom. Used this plainly it can name any people, Israel included: Israel is called a nation, and so is Samaria, Egypt, or the many nations of the earth.
The New Testament inherits a Jewish habit of speech that reshapes the word in the plural. Israel saw the world split in two: itself, the covenant people, and everyone else, the nations. So the plural the nations came to mean specifically the non-Jewish peoples, the Gentiles, and that became the word's center of gravity in the early church, above all in the mission to bring the gospel to them.
From that same Jewish vantage point the plural also picked up a sharper edge. Because the surrounding peoples did not know the God of Israel, the nations could be shorthand for the pagans, those who live without that God and worship idols. Whether the word is neutral, simply ethnic, or loaded with that religious judgment depends on the verse, which is why the same form can describe a people on a map and a way of life the writer warns against.
These are not separate words but one word seen from different angles: the neutral body of people, that body as it stands outside Israel, and that body as it stands outside Israel's God.