Being sexually aroused by pain, applied to the erogenous zones specifically, such as the genitals.
Becoming sexually aroused, at having pain inflicted upon them. It is a form of deriving pleasure from actual pain, in which the pain is processed internally as being pleasurable, rather than hurtful.
[From the Greek 'algos' meaning pain, and 'lagnia' meaning lust]
In the earl 1900's, Havelock Willis researched the connection between how the Brain processed certain emotions attached to pain, finding that in some people, they actually processed pain as being pleasurable, and that it wasn't a quirk, or result of deviant desires.
Not to be confused with sadomasochism, this is not the mental need to find pain, for obtaining sexual gratification. It is simply a mental processing of the feelings differently, where pain is treated as pleasure.
It does not mean one is seeking painful experiences, to satisfy these feelings, which many believe Sadomasochists do.
Noteworthy: Modern research has shown, through the use of MRI's and computer models, showing the neuron firing process, that people with this disorder simply experience pain differently, due to how the brain processes.
It is more of a psychological reaction to pain, while sadomasochism is more of a psychological desire for the pain, and humiliation, in order to be satisfied sexually.
Some modern research results consider such a condition being a result of a DNA miscue, or defect.