Masochism is a form of sexual gratification in which an individual is sexually aroused by either the threat or enactment of pain or humiliation and degradation. This might involve whipping, beating, bondage, submission to and by a more dominant sexual partner.
A paraphilia in which pleasure is derived from physical or psychological pain inflicted on oneself or by others. When it is consciously sought as a part of the sexual act or as a prerequisite of sexual gratification, it is classified as a paraphilia.
The desire for and ability to process intense/painful sensation for enjoyment and sexual arousal. Sometimes it is indulged in for the endorphins alone, without arriving at any sexual act. Generally an activity for two people.