That process in the soul of a sinner by which he is transferred from the state of enmity with God to the friendship of God.
As an act it is the gratuitous work of God alone; but in the adult cooperation, moral preparation, e.g., faith, contrition, etc., is required.
As a habit it is the continued possession of sanctlfying grace.
The removal of sin and the infusion of grace constitute one and the same act.
Removal of original sin by Baptism is called first justification; forgiveness, in the Sacrament of Penance, of mortal sin committed after Baptism, is called second justification.
By an act of Perfect contrition man can be justified before actual reception, but not without the desire, of the Sacrament.
New Catholic Dictionary