Literally 'from the same womb' — a brother by blood. In the NT, it expands to include spiritual brothers and sisters, fellow members of the faith community. Adelphos became the standard address among early Christians, reflecting their radical new family identity in Christ.
The early church's use of adelphos was revolutionary — it dissolved ethnic, social, and economic barriers. Slave and master, Jew and Gentile became 'brothers.' Paul's letter to Philemon embodies this: Onesimus is 'no longer as a slave, but better than a slave, as a dear brother' (Philemon 1:16). Jesus Himself redefined family: 'Whoever does God's will is my brother' (Mark 3:35).