The standard Greek word for God, over 1,300 times in the NT. In classical Greek, used broadly for any deity. The NT transforms it through Hebrew heritage: theos is the one true God of Israel — Creator, Sustainer, and Redeemer revealed definitively in Jesus Christ.
The bold claim that Jesus is theos (John 1:1; 20:28; Romans 9:5) was revolutionary. Paul's Areopagus speech (Acts 17) uses theos to connect with Greek philosophical theism while subverting it with the gospel.
Both theos and Elohim (H430) carry plural resonance. Thomas's confession (John 20:28) and John's prologue establish Christ's divinity using the most theologically loaded word available. The early church councils were essentially defining what theos means when applied to Father, Son, and Spirit.