The preservation of the state of grace until death. It is perfect if baptismal innocence is not lost; but this is not necessary, since grace lost by mortal sin can be regained.
The power of perseverance is the sum total of all the spiritual means at man's disposal to continue in grace, provided he cooperates.
This, however, does not assure actual perseverance.
Actual final perseverance implies two elements: the faithful, continual use of the means of grace; and the condition that the person die in the state of grace.
The Council of Trent calls it a special Divine gift, because it is neither included in sanctifying grace, nor is it the result of the power of perseverance.
Practically it is a series of efficacious graces.
It is not God's grace alone, nor the human will alone, but a combination of both, God's benign, continual providence, guiding, protecting, assisting, and man's faithful cooperation with Divine grace.
Sacred Scripture, the Holy Fathers, and Doctors of the Church represent final perseverance as the fruit of prayer.
New Catholic Dictionary