Objection 1. It would seem that contrition is not an act of virtue. For passions are not acts of virtue, since "they bring us neither praise nor blame" (Ethic. ii, 5). But sorrow is a passion. As therefore contrition is sorrow, it seems that it is not an act of virtue.
Objection 2. Further, as contrition is so called from its being a crushing, so is attrition. Now all agree in saying that attrition is not an act of virtue. Neither, therefore, is contrition an act of virtue.
On the contrary, Nothing but an act of virtue is meritorious. But contrition is a meritorious act. Therefore it is an act of virtue.
I answer that, Contrition as to the literal signification of the word, does not denote an act of virtue, but a corporeal passion. But the question in point does not refer to contrition in this sense, but to that which the word is employed to signify by way of metaphor. For just as the inflation of one's own will unto wrong-doing implies, in itself, a generic evil, so the utter undoing and crushing of that same will implies something generically good, for this is to detest one's own will whereby sin was committed. Wherefore contrition, which signifies this, implies rectitude of the will; and so it is the act of that virtue to which it belongs to detest and destroy past sins, the act, to wit, of penance, as is evident from what was said above (Sent. iv, D, 14, 1, 1; III, 85, 2,3).
Reply to Objection 1. Contrition includes a twofold sorrow for sin. One is in the sensitive part, and is a passion. This does not belong essentially to contrition as an act of virtue, but is rather its effect. For just as the virtue of penance inflicts outward punishment on the body, in order to compensate for the offense done to God through the instrumentality of the bodily members, so does it inflict on the concupiscible part of the soul a punishment, viz. the aforesaid sorrow, because the concupiscible also co-operated in the sinful deeds. Nevertheless this sorrow may belong to contrition taken as part of the sacrament, since the nature of a sacrament is such that it consists not only of internal but also of external acts and sensible things. The other sorrow is in the will, and is nothing else save displeasure for some evil, for the emotions of the will are named after the passions, as stated above (Sent. iii, D, 26, 1, 5; I-II, 22, 3, ad 3). Accordingly, contrition is essentially a kind of sorrow, and is an act of the virtue of penance.
Reply to Objection 2. Attrition denotes approach to perfect contrition, wherefore in corporeal matters, things are said to be attrite, when they are worn away to a certain extent, but not altogether crushed to pieces; while they are said to be contrite, when all the parts are crushed [tritae] minutely. Wherefore, in spiritual matters, attrition signifies a certain but not a perfect displeasure for sins committed, whereas contrition denotes perfect displeasure.
Source: Summa Theologica (New Advent)