Entromos (ἔντρομος, G1790) means trembling, quaking with fear, terrified. From en (in) + tromos (trembling). It appears in Acts 7:32 (Moses trembled before the burning bush), Acts 16:29 (the Philippian jailer 'trembling with fear' after the earthquake), and Hebrews 12:21 (Moses said 'I am exceedingly afraid and trembling' at Sinai).
The trembling of entromos marks every decisive encounter with the holy in Acts and Hebrews. Moses trembles at the burning bush (Acts 7:32) and at Sinai (Hebrews 12:21) — encounters with the undiluted presence of God produce involuntary reverence. The Philippian jailer trembles (Acts 16:29) not before God's direct presence but before the unmistakable evidence of His power — an earthquake that freed prisoners and did not cause them to flee. His trembling is the beginning of conversion: 'Sirs, what must I do to be saved?' (Acts 16:30). Hebrews 12:18–21 contrasts the terrifying trembling of Sinai with the glorious gathering of Mount Zion — yet even in Zion, Hebrews 12:28 exhorts: 'let us be thankful, and so worship God acceptably with reverence and awe.' Entromos fear is not eliminated in the New Covenant — it is transformed from servile terror into reverent awe, the appropriate posture before the living God.