Not sin in the ordinary sense of a transgression of the law of God which one commits deliberately.
It is one of the consequences of the sin of our first parent, Adam. As father of the human race, he was endowed with immortality, with reason and will in perfect control of the lower appetites, and with Divine grace enabling him to know and serve God in a manner far beyond the capacity of his natural powers, and therefore in a state above nature: the supernatural state.
Through him these endowments were to be transmitted to the entire human race.
When by his sin he lost them for himself, they were lost also for his descendants.
Now the loss or privation of Divine grace, the chief consequence of sin, means the privation of the supernatural goodness to which God destined us, and therefore it is called our original stain or sin.
Other consequences of Adam's sin are death, and concupiscence, or the rebellion of our lower appetites against reason and will.
This concupiscence, though often the occasion of sin, is not in itself sinful; it is not original sin, as the early Protestants held, nor does it consist, as Luther believed, in such a decadence of our nature as to leave our reason incapable of understanding, our will without freedom, and our whole nature evil.
Original sin does not so corrupt our natural powers as to render them incapable of natural virtues: it deprives us of the grace needed for virtues beyond our natural powers.
All this is based on Holy Scripture, particularly on Saint Paul's Epistle to the Romans 5:
"Wherefore as by one man sin entered into this world, and by sin death; and so death passed upon all men, in whom all have sinned."
It is the subject of the Fifth Session of the Council of Trent.
New Catholic Dictionary