The literal meaning of Justification is to be declared righteous in the sight of God. It is an act of God, freely given or accounted to those who believe in Jesus Christ, the Son of God. By no other means, one could be justified and be righteous before God. So, it is grounded solely on the work of Christ. This righteousness of Christ is imputed in us, as we receive him by faith.
The Protestant understanding of justification is rejected by the Roman Catholics. They believe that believers also need to do good work. Faith in Jesus alone is not enough to be justified. For this reason, man must work for his own salvation. They mixed up Justification with Sanctification. Nevertheless, we do good work not be justified but it is the sole purpose and goal of Justification. Therefore, the righteousness of Christ is not infused in us but imputed in us.