Incest is usually defined as a sexual relationship between people who are too closely related by blood. 'Too closely' in the vast majority of cultures means brothers and sisters, parents or grandparents and children. There have been a very few exceptions to this such as ancient Egypt, where brothers were sometimes married to their sisters, but these are very rare.

Cousin marriage isn't usually seen as incest, although first cousin marriage is illegal in some US states. This is usually on the grounds that children of close relatives are more likely to have genetic problems, though this has been disputed lately.

Several countries, including France and Japan, allow incest but not marriage between a brother and sister, while in Brazil marriage is sometimes allowed between an uncle and a niece. Sweden is the only country that actually allows marriage between half siblings in specific cases, but special permission has to be sought.