It's a surprisingly difficult question to answer. Socrates spends the whole of the Meno dialogue discarding various definitions. When I discussed it with a study group we were surprised how difficult it was to come up with a definition that would suffice and not set false conditions. (without cheating by looking up Aristotle or Aquinas).
Aquinas calls it a habitus operativus bonus; Father Hardon translates this as a good habit bearing on activity. This site calls virtue an operative habit productive of good works.
Every word is important.
- "Habit" means that virtue doesn't reside in a single act; it has to be repeated; there has to be a "disposition" or will to do the good thing consistently.
- "Good" -- well, that is obvious. An evil habit takes away from virtue and does not add to it.
- "Operative" or "productive of works" -- that is, it's not enough to simply have good intentions. They have to be carried out in practice according to one's ability. (the part about one's ability is important because obviously a shut-in wouldn't be able to go out as a missionary, say, but she could still be perfect in virtue by doing what she could do well).
The Catholic Encyclopedia elaborates the Thomist definition:
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.That last paragraph is interesting because it seems to imply that a habit (in the sense of virtue) isn't so much a matter of a particular action as a quality disposing one toward a particular KIND of action.
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.
Father Hardon writes:
Habits reside in the faculties as stable dispositions or "hard to eradicate" qualities that dispose the faculties to act in a certain way, depending on the type of habit. If the habit is acquired it gives the faculty power to act with ease and facility;....
Natural or acquired habits result from repeated acts of some one kind; they give not the power to act, but the power to act readily and with dexterity. Thus in the natural order, the faculty without the habit is simple power to act, the faculty with the habit is power to act with perfection. Since custom is parent to habit, it is called second nature. Faculty is like first nature, and habit the second.
Not every habit is a virtue, but only one that so improves and perfects a rational faculty as to incline it towards good -- good for the faculty, for the will and for the whole man in terms of his ultimate destiny.