Definitions of Justification
Don's definition: God judicially decreeing that a sinner has satisfied His requirement for perfect righteousness.
Dictionary.com: (theology) The act of God whereby humankind is made or accounted just, or free from guilt or penalty of sin.
Dr. John MacArthur: The judicial decree of God in declaring innocent the sinner who trusts on Jesus Christ.
Those God effectually calls he also freely justifies.1 He does this, not by infusing righteousness into them but by pardoning their sins and accounting and accepting them as righteous.2
1 - Rm 3.24, 8.30
2 - Rm 4.5-8, Eph 1.7
[1689 London Baptist Confession of Faith, Chapter 11, Paragraph 1]