The Hebrew verb achad (אָחַד) means to go in one direction, to be united, or to become one. It is the verbal root of the cardinal number echad (H259, one) and implies unity achieved through convergence or joining together.
The Shema — Hear O Israel, the LORD our God, the LORD is one (echad) — proclaims the unity of God as the foundation of Israel's faith. Jesus prayed that His followers would be one as He and the Father are one (John 17:21). Biblical unity flows from relationship with the One God and finds fulfillment in the one Body of Christ.