There seem to be two arguments about torture going on. One is whether torture is objectively immoral (and no, I don't distinguish between "objectively immoral" and "intrinsically immoral"). The other is whether particular acts constitute torture.
I've already written my piece on the objective immorality of torture, but as several commenters have noted, saying, "Torture is evil," isn't very helpful if you don't know what is and is not torture.
A Washington Post editorial reports that "the Army's interrogation procedures -- which were unclassified -- established this simple and sensible test: No technique should be used that, if used by an enemy on an American, would be regarded as a violation of U.S. or international law." A nicely put Golden Rule-type definition, even if in terms of specifics it leaves things up to U.S. and international law.
In the comments below, I offered "an act the object of which is the infliction of physical or mental torment" as a working definition, and so far people have been distracted enough by what the object of an act is that they haven't asked me what "torment" is. Which is fine by me.
But while I won't pretend I can come up with a universally applicable definition of the "torture" that is objectively evil, I do think we can get a good general idea of what it is by looking at the language of the texts that proscribe it. Torture is "contrary to respect for the person and for human dignity;" it "violates the integrity of the human person."
Furthermore, I'd suggest it connotes circumstances in which the torturer has more or less complete and persistent control over the victim and the torment is premeditated.
So an act of torture is an act that
has as its object the physical or mental torment -- okay, pain and anguish -- of a person;
constitutes a violation of the victim's personal integrity or human dignity;
manifests the torturer's control over the victim
I'm sure this can be refined in all sorts of ways. In particular, Gaudium et Spes says #1 implies #2, but as we all know there are forms of humiliation that are clear violations of the victim's dignity but aren't so clearly painful and anguishing.