The rack, strapato, iron maiden, hot coals, pincers under the fingernails, cat-o-nine tails, bullwhip, are all examples of torture.
They are examples of torture because they leave lasting injuries and scarring on the body. Mental scarring is a nice simile but if you can overcome it with therapy, it isn't a permanent scar.
Waterboarding does not leave scars.
The UN definition of "torture" is so broad that not refunding someone their money after they walk out of the latest Rosie O'Donnell movie is torture.
...any act by which severe pain or suffering, whether physical or mental, is intentionally inflicted on a person for such purposes as obtaining from him, or a third person, information or a confession, punishing him for an act he or a third person has committed or is suspected of having committed, or intimidating or coercing him or a third person, or for any reason based on discrimination of any kind, when such pain or suffering is inflicted by or at the instigation of or with the consent or acquiescence of a public official or other person acting in an official capacity. It does not include pain or suffering arising only from, inherent in, or incidental to, lawful sanctions.
—UN Convention Against Torture
So now that the broadest definition of torture has been given, let us look at the correct definition.
1 a: anguish of body or mind : agony b: something that causes agony or pain
2: the infliction of intense pain (as from burning, crushing, or wounding) to punish, coerce, or afford sadistic pleasure
Waterboarding is uncomfortable, but it is not torture. Real torture is much more severe.