There seems to be some sort of silent agreement on both sides that this is an impossible thing to do. I’m missing something here since the action doesn’t seem that hard to define: the use of physical pain to break the will of another, where “breaking the will” (which can mean more than one thing) means “breaking ones self possession”. The definition manifests why such an action would be intrinsically evil, since to be in possession of ones own power to choose or of ones own will is necessary for human dignity. A man is a lord of his action, so much so that to attempt to break this lordship is, in a very real sense, worse than murder. It is the attempt to kill what is most of all human in a human being.
Puzzling, as this is a definition that those adhering to a "liberal" version of Natural Law might accept, but it's a poor one, as far as I can tell -- how does one distinguish torture from the deterrent effect of law or legitimate coercion, for example police officers using pain compliance on those actively resisting arrest? (See my previous posts on this topic.)
Torture (or fear) does not destroy voluntariness (see the treatment of voluntary, involuntary, and non-voluntary) , though in some instances fear can diminish responsibility.