Anyone who reads Catholic and Enjoying It! knows that the question of defining torture is not a simple one. I haven't tried to answer it myself, for a few reasons.
For one, as soon as a definition is offered, the discussion becomes one of the adequacy of the offered definition. It becomes, in other words, a linguistic exercise rather than a moral one.
For another, there seem to be a lot of people eager to apply this invalid syllogism: "Act X is not torture. Torture is not moral. Therefore, act X is moral." I think we're better off working through this fallacy as it stands before looking at whether the first premise is true. The fundamental moral principle is not, "Do good and avoid torture," it's, "Do good and avoid evil."
Nevertheless, being asked to come up with the general kind of thing torture is and the specific difference between torture and other acts that are the same general kind of thing seemed reasonable. So I took that statement from Gaudium et Spes 27:
Furthermore, whatever is opposed to life itself, such as any type of murder, genocide, abortion, euthanasia or willful self-destruction, whatever violates the integrity of the human person, such as mutilation, torments inflicted on body or mind, attempts to coerce the will itself; whatever insults human dignity, such as subhuman living conditions, arbitrary imprisonment, deportation, slavery, prostitution, the selling of women and children; as well as disgraceful working conditions, where men are treated as mere tools for profit, rather than as free and responsible persons; all these things and others of their like are infamies indeed. [emphasis added]
And proposed this definition of torture:
Torture is an act belonging to the genus of acts that violate the integrity of the human person; its specific difference is that its object is to inflict torments on body or mind.
What surprised me with this is that, though I left off the "attempts to coerce the will itself" bit, I don't really miss it.
Just recently, I'd begun to think of torture as being of two types: one that causes pain, and one that attacks the free will. Now, though -- and coming from the position that the debate in the public square should be, not "What acts are torture?," but, "What acts are moral?" -- I think it might be possible to get somewhere with the above definition, and set aside for a time consideration of the act belonging to the genus of acts that violate the integrity of the human person, the specific difference of which is that its object is to coerce the will itself.