Pain: Undesirable, negative feeling or feelings, related to various kinds of experiences and states of the body or the mind.
Pain, like pleasure, is hard to define adequately, and a fundamental quality of human experience.
Much of what could be said about pains - such as that they come in different kinds - is quite similar to what can be said about pleasure, to which I refer the reader, except of course that pains normally are judged negatively and avoided, usually, if they are not part of a condition to achieve something valuable.
But this is not true of all kinds of pain, since it seems some find a sort of pleasure in exhausting, difficult or dangerous sports (running the marathon, climbing mountains, bungy-jumping), whereas some mild pains, like those involved in teasing or tickling, may be found pleasurable or may be found to cause pleasure.
Then there is also a rather paradoxical quality to pain, especially strong and excruciating pain. Namely: While it makes sense to assume that - at least some physical - pain exists to warn the body about what might damage it if it continues some activity, it seems a rather serious faulty 'design' that many pains can be extra-ordinarily severe, while they may relate to little real damage (tooth-ache; pulling out a nail). Life certainly would be less unpleasant if instead of pain one would have a mental warning system that just warned, instead of also hurting. (Or, as probably many men and women have concluded through the centuries: I could have given the dear Lord - or our divinities - some rather useful advice concerning the design of the world and of human beings, simply based on the experience of what it feels like to be human.)
And related to this point there is the fact that whereas one may doubt many things, including one's own existence ('am I not a brain in a vat?'), it seems very difficult or impossible to doubt that one is in pain, if one is.
Furthermore, it would seem - though I do not know whether this has ever been seriously investigated, nor do I know how one could do this - as if painful emotions (fear, threats, physical pain) are stronger motivators than pleasurable emotions, possibly because the former help one to keep alive, while the latter merely make the life one leads more pleasant. Indeed, it would seem as if people do more to decrease pain than to increase pleasure: Pain is always bad and to be avoided, but pleasure is satisfactory also in small amounts.
For more, see pleasure.