Dikaiōsis appears only twice in the NT (Romans 4:25; 5:18) but is the noun form of one of Paul's most important theological terms. It comes from dikaioō (G1344), 'to justify, declare righteous,' and dikaios (G1342), 'righteous, just.' While dikaiosynē (G1343, righteousness) describes the status, dikaiōsis describes the act or process of being declared righteous. Romans 4:25 says Christ 'was delivered over to death for our sins and was raised to life for our dikaiōsis' — justification is grounded in both the death and resurrection of Jesus.
The doctrine of justification by faith (dikaiōsis ek pisteōs) was the theological heart of the Protestant Reformation, but it was Paul's discovery — or rather, his unpacking of Genesis 15:6 ('Abraham believed God, and it was credited to him as righteousness'). Justification is forensic (a legal declaration), relational (restoring covenant standing), and transformative (producing real righteousness through the Spirit). Romans 5:18 presents the cosmic scope: 'just as one trespass resulted in condemnation for all people, so also one righteous act resulted in justification and life for all people.' Christ's obedience reverses Adam's disobedience; his condemnation opens the way to our dikaiōsis.