The sexual pleasure derived from either the inflicting of extreme pain, or the receiving of extreme pain, or both.
Sadists enjoy giving pain, it does NOT necessarily mean they also derive sexual satisfaction or pleasure from it.
Masochists enjoy receiving pain, again this does NOT always mean sexual pleasure.
The terms Sadist and Masochist were originally technical terms, used to describe a psychological features of a more detailed mental illness. The words themselves were taken from the authors Marquis de Sade and Leopold von Sacher-Masoch.
In 1843 Hungarian physician Heinrich Kaan published an essay (writing) in which he linked the sins, as contained in Christian Doctrine, into medical terms.
At the time, theological terms such as 'perversion, aberration, and deviation' were restricted for use in Scripture or Religious connotations. He brought them into wider use within the Medical profession.
German psychiatrist Richard von Kraft-Ebbing first coined the word sadomasochist in 1890.
Sigmund Freud in 1905 published his 'Three Papers on Sexual Theory' in which he attributed sado-masochism to a disease, generated by incorrect development of one's childhood psyche.
There is disagreement as to the extend in which a sadist and masochist are compatible. Many who practice this form of SandM claim to be switchable, meaning that they can achieve pleasure either from the giving of pain, or the receiving of pain.
Kraft-Ebbing and Freud also claim that there is an element of cruelty that is part of this believe, which later analysts disagree with.
They claim that the two are not really able to cross over, and that the aspect of cruelty is not a part of the current SandM sub culture of what is categorized under the BDSM Community.
SandM (sadomasochism) has a bad rap attached, which has led to the development of other terms within the BDSM community. Those terms are the D's (Discipline) and the Bondage components of the larger community.
It is argued that SandM is not about cruelty, but about satisfying another's pleasure needs, for pain. That the administering of pain, or its reception, is done out of love, rather than out of some desire for cruelty.
Practice (Associated Acts):
Havelock Ellis made the argument that both Sadist and Masochists are interchangeable, that they derive the same pleasures, from different attributes of 'the giving of pain'.
He considered them complimentary, in that each needed the other to complete their needs.
In fact, he argued that in many cases it was more about control, than just the pain factor, and further claimed that in many cases, it was the Masochist who was the person in control, rather than the sadist, the giver of the pain.
Noteworthy: SandM has two actual meanings, that can be related, but not necessarily so.
SandM can refer to sadism and masochism. The giving of pain, and the receiving of pain.
It can also refer to Slave and Master which does not derive pleasure solely from the giving of pain, or the receiving of pain.
Noted Author and Psychiatry advisor to the New York Daily News, Doctor Joseph Merlino has stated that in a consensual agreement between participating sadomasochists is NOT a psychological disorder.
In 1994, the publication of the Diagnostic and Statistical Manual of Mental Disorders no longer lists Sadomasochism as being a mental disorder of sexual preferences, but as illnesses of themselves. The new criteria for diagnosing these illness became more refined.
Explanations for why some enjoy being Masochists, is basically an individual choice.
For many it is the appeal of being helpless, of not having to make any decision, of being under someone else's control rather than their own. It can also be a way to relieve the daily stress of their otherwise 'normal' life.
As well, it can be a sort of release of their guilt feelings, or atonement for it, by submitting to be under another's absolute control.
Sadists may enjoy the power that administering pain can give them. A sort of being in charge, when they may normally not be.
It also creates a sense of authority for them, that enhances the pleasure they receive.
Again, it is subjective to each individual.
Gilles Deleuze believes that sadomasochists are not the same, or should be lumped together.
His theory is that the Masochist is more into the control aspect, based on 'the contract' in which they force another to become cold, callous.
The Sadist on the other hand, is more about 'The Law' in that one person is placed above another, as a natural order of things. It is then re-enforced by the giving of pain.