Incest is sexual activity between close family members. Incest is considered taboo, and forbidden (fully or slightly) in the majority of current cultures. The precise meaning of the word varies widely, because different cultures have differing notions of "sexual activity" and "close family member." Some cultures consider only those related by birth, while others include those related by adoption or marriage. Some prohibit sexual relations between people who grew up in the same household, while others prohibit sexual relations between people who grew up in related households.
Incest can occur between same-sex as well as opposite-sex relatives. It can also occur between related children as well as between parents and their children. In addition, there have been cases of incest between adult relatives.
Incest between close blood-relations is a crime in most Western nations, as well as in those nations that were colonised by the West, although again the extent of the definition of "close" varies. However, since incest is an intrapersonal act that takes place in private, it is a difficult law to be enforced. There are wide differences between nations as to how serious the crime of incest is. In some countries, such as Australia, incest is a serious indictable offence, while in others it is a minor crime with much less serious consequences.
Actual incest is generally a taboo subject, and is considered a hard limit by practitioners of SSC BDSM. Incest fantasies, however, may be within a persons play limits.