The Greek verb symphōneō (συμφωνέω) means to sound together harmoniously, and thus to agree or come to an agreement. It is the root of the English word 'symphony.' In the New Testament it is used for parties coming to an agreement (Matthew 18:19; 20:2,13) and for a discordant note that sounds with the devil (Acts 5:9).
Matthew 18:19 contains one of Jesus's most precious promises about prayer: 'If two of you on earth agree (symphōnēsōsin) about anything they ask for, it will be done for them by my Father in heaven.' The context is the gathered community with Christ present (v.20). The agreement is not merely intellectual consent but a harmony of will — the symphonic prayer of believers aligned with God's purposes. Acts 5:9's use for Ananias and Sapphira's conspiracy — 'How could you agree together to test the Spirit?' — shows the dark counterpart: human scheming in concert against God's truth.