To justify is to identify a subject as standing in a right relation to the governing standard.
In salvation, justification is God's true and authoritative judgment that the sinner who believes upon Him stands in right relation to God because he is found in Christ.
It is forensic without being a legal fiction: God's judgment corresponds to the believer's real position in the Righteous One. Union establishes the sinner's position in Christ; justification identifies that position as right; sanctification progressively brings his character and conduct into correspondence with the position he already occupies.
See in Scripture: Romans 3:24–26; Romans 5:1; Philippians 3:9