A king, sovereign ruler. Basileus is the NT's word for earthly kings (Herod, Caesar) and supremely for Christ — the King of kings. The related noun basileia (kingdom) appears even more frequently, as Jesus' central message was the kingdom of God.
The entire NT narrative is about kingship: who rules, and by what right? Pilate's question 'Are you the king of the Jews?' gets Jesus' paradoxical answer: 'My kingdom is not of this world.' The sign on the cross — 'King of the Jews' — was meant as mockery but proclaimed truth. Revelation culminates with Christ as 'King of kings and Lord of lords' (19:16). His kingship is exercised through service, sacrifice, and love — a radical redefinition of power.