From Catholic Encyclopedia:
According to its etymology the word virtue (Latin virtus) signifies manliness or courage. “Appelata est enim a viro virtus: viri autem propria maxime est fortitudo” (“The term virtue is from the word that signifies man; a man’s chief quality is fortitude”; Cicero, “Tuscul.”, I, xi, 18). Taken in its widest sense virtue means the excellence of perfection of a thing, just as vice, its contrary, denotes a defect or absence of perfection due to a thing.
In its strictest meaning, however, as used by moral philosophers and theologians, it signifies a habit superadded to a faculty of the soul, disposing it to elicit with readiness acts conformable to our rational nature.
“Virtue”, says Augustine, “is a good habit consonant with our nature.” From Saint Thomas’s entire Question on the essence of virtue may be gathered his brief but complete definition of virtue: “habitus operativus bonus”, an operative habit essentially good, as distinguished from vice, an operative habit essentially evil.
Now a habit is a quality in itself difficult of change, disposing well or ill the subject in which it resides, either directly in itself or in relation to its operation. An operative habit is a quality residing in a power or faculty in itself indifferent to this or that line of action, but determined by the habit to this rather than to that kind of acts.
Virtue then has this in common with vice, that it disposes a potency to a certain determined activity; but it differs specifically from it in that it disposes it to good acts, i.e. acts in consonance with right reason. Thus, temperance inclines the sensuous appetite to acts of moderation conformably to right reason just as intemperance impels the same appetite to acts of excess contrary to the dictates of our rational nature.