A Greek noun meaning nation, people, race. In the plural (ta ethnē), it specifically refers to the nations, the Gentiles — all peoples outside of Israel. It is the root of the English word 'ethnic.' In the New Testament, it is a theologically loaded term marking the boundary between Jews and non-Jews — a boundary that the gospel dramatically breaks down.
The inclusion of the ethnē (Gentiles/nations) is one of the New Testament's central theological dramas. God's promise to Abraham was that 'in you all the nations shall be blessed' (Genesis 12:3; Galatians 3:8) — the Gentile mission was always the plan. Yet Acts records the shock when the Spirit falls on Gentiles (Acts 10:45). Paul becomes the 'apostle to the Gentiles' (Romans 11:13) and declares the mystery: that ethnē are 'fellow heirs' with Israel (Ephesians 3:6). Revelation 7:9 depicts the consummation — every nation, tribe, people, and tongue before the throne. The gospel is not the narrowing of God's people but the explosive widening to include all nations.