Recently Christopher at Against the Grain asked me to take a stab at defining torture, so I'll do my best. The Magisterium has not given us a definition, so individual thinkers are at this point left to their own devices to come up with one.
In producing a definition, I'd like to start with two basic parameters:
Parameter 1: The definition should correspond as much as possible to our pre-reflective sense of what constitutes torture.
Parameter 2: The definition should point to something that is intrinsically evil.
The reason for Parameter 1 is that you should always start with a commonsense understanding of a term in attempting to give it a technical definition. The technical definition should capture as much of the commonsense understanding as possible. Otherwise you get linguistic chaos.
The reason for Parameter 2 is that I think the Magisterium would want the word "torture" used in a way that points to something intrinsically evil.
In Veritatis Splendor, John Paul II quoted a list of social evils--including torture--from Gaudium et Spes and seemed to apply the label "intrinsically evil" to this list. This does not strike me as sufficient to settle the question, though, for as His Awesomeness Cardinal Dulles has pointed out, John Paul II's use of this passage from Gaudium et Spes appears to have important unstated qualifiers and thus some of the items on the list (e.g., deportations) do not on their face appear to be intrinsically evil without further qualification. The possibility is thus raised (and I view Dulles's article as turning the possibility into a probability) that the pope was speaking in a general rather than a technical way and without further qualification we cannot simply say that every item on the list is intrinsically immoral.
So I don't think Veritatis Splendor is decisive on this question. Instead, I think that the evolution of the word "torture" will unfold in such a way in the future that the Magisterium will want it used of intrinsic evil.
The reason is this: The history of an institution can constrain the way that institution uses words. A classic example of this is the development of the Roman Empire out of the Roman Republic. Prior to the Republic, the Romans had kings, and getting rid of the kings and establishing a Republic was one of their proudest achievements. Consequently, Romans of that era could never allow themselves to have a king. This meant that, even when the Caesars were given king-like powers and were clearly functioning as kings (as in "We have no king but Caesar"), the Romans themselves couldn't call them kings. So Augustus Caesar asked instead for the title "Imperator" ("commander"), which is how Rome got its emperors.
Americans, because they threw off the rule of both a king and an empire, likewise can have neither kings nor an empire. Even if the presidency were one day morphed into a functional monarchy, we couldn't call it that. Similarly, even if we acquire a functional empire (I don't perceive us as having one, though I know others do), we will not in the foreseeable future get to the point that you have Congress and the president referring to "our empire" the way the Romans did.
A similar situation applies with regard to the history of the Church: The fact that Church authorities once used torture, in keeping with the legal custom of the day in secular society, is a matter of intense shame. To make it clear that this chapter of history is definitively over and that the Church has thoroughly broken with and renounced this practice, it will want to issue vigorous condemnations of torture--as indeed, it has.
And that constrains the way that the word "torture" is likely to be used.
If "torture" is not restricted to things that are intrinsically immoral (always wrong) then the Church--or at least moral theologians--would be put in the position of having to say that sometimes torture is not wrong.
Given its history, that is not something the Church will want itself--or its moral theologians--to be saying. The Church would be a lot happier if Catholic thinkers proposed definitions of "torture" that point to things that are intrinsically immoral, so I will seek to develop such a definition.
The inclusion of Parameter 2 may mean that our definition may not capture the full range of what has historically been called torture. Historically, the word has not been subject to the requirement of Parameter 2, and as a result, our commonsense understanding of torture likely covers things that are not intrinsically wrong but only extrinsically wrong.
This is normal whenever you try to give a technical definition for something that previously has only had a commonsense understanding: The technical definition never corrsponds fully to the previous, non-technical usage.
There thus may be some things that would historically have been thought of as torture that are not intrinsically wrong and thus can be justified in at least extreme circumstances. Indeed, this seems to be a large part of what is prompting the present theological and social debate about torture: Some people feel that due to the War on Terror we are in a situation in which some things that would historically have fallen under our commonsense understanding of torture (Parameter 1) may now be justified, but there is also a contemporary impulse to say that torture is always wrong (Parameter 2).
In harmonizing the two parameters, our resulting definition will attempt to capture as much of the traditional understanding of torture as possible, but only those elements that are intrinsically immoral. If something can in some circumstances be justified then we won't call it torture.
And now for,
THE BIG RED DISCLAIMER: Just because something can be justified in at least some circumstances DOES NOT MEAN THAT WE ARE IN THOSE CIRCUMSTANCES and thus DOES NOT MEAN THAT YOU CAN GO AHEAD AND DO IT. There may be many things that, while not intrinsically evil, are extrinsically evil in the vast majority of circumstances and cannot be done as part of the War on Terror.
One note about the future evolution of the term "torture": If Catholic moral theology and/or the Magisterium follow the course I suggest that they will and begin using torture to refer to things that are intrinsically evil then this will mean that its technical definition of torture will begin to diverge from the popular, commonsense understanding.
That's not surprising. Technical definitions always diverse from popular ones since the populus isn't composed of theological experts.
I see the situation as analogous to the use of the term "theft." There is a popular understanding of the term "theft" that would include taking food from someone who has plenty if you are starving and cannot buy food. According to the popular usage, that would count as theft, and an ordinary person might say, "Sometimes theft is okay." The Church does not want to say that sometimes theft is okay, and so it defines the sin of theft in such a way that this is precluded (i.e., taking property against the reasonable will of its owner). The Church would thus say that theft is always wrong, but taking food in the above circumstances does not count as the sin of theft.
In the same way, there may be things that would count as torture under the popular understanding and yet be justified, leading an ordinary person to want to say "Sometimes torture is okay." But the Church will not want to say that and so--if my thesis is correct--it will instead define torture such that those things which are potentially justifiable do not count as torture.
When the Magisterium might do that, of course, I have no idea. Maybe not for centuries--if ever--but I suspect that Parameter 2 will be a factor in the future evolution of the term.