The Hebrew verb tsadaq (צָדַק) means to be righteous, to be just, to be in the right. In the causative stem (Hiphil), it means to justify, to declare righteous, to vindicate. Its root relates to conformity with a standard — whether of God's law, covenant loyalty, or court judgment. The related noun tsaddiq (H6662) means "righteous one" and the noun tsedeqah (H6666) or tsedeq (H6664) means "righteousness." These words appear hundreds of times in the Old Testament and form the foundation for the New Testament's doctrine of justification.
In forensic and legal contexts, tsadaq describes a court declaring someone "not guilty" or "in the right." This is the background for Paul's doctrine of justification: God, the righteous Judge, declares the sinner righteous — not because of their merit, but because of the righteousness of Christ imputed to them by faith (Romans 3:26; 4:5). The Old Testament anticipates this in Abraham: "Abram believed the LORD, and he credited it to him as righteousness [tsedeqah]" (Genesis 15:6). Isaiah 53:11 predicts the Servant who will "justify many" by bearing their iniquities. The Psalms cry out for God to vindicate the righteous (Psalm 17; 26). Tsadaq is not merely moral behavior but a right standing before the Creator — which is why justification is both the heart of the gospel and the ground of all Christian ethics.