Ach means brother, kinsman, or fellow countryman. Appearing over 600 times, it covers biological brotherhood, tribal kinship, and covenant relationships between allies and those bound by shared loyalty.
Brotherhood in Scripture carries deep covenantal significance. From Cain's question 'Am I my brother's keeper?' to the command to love one's neighbor, ach establishes mutual responsibility within God's community and reflects the ideal of unity among God's people.