According to an Encyclopedia (link), incest is (latin "incestus": contaminated, sexually immoral, an older word for incest is bloodshame) a term for sexual activity between persons who are related by blood and/or are not legally allowed to marry. Sexual intercourse with a descendant or between full brothers and sisters is punishable according to the law in most countries. In a historical perspective the penalty for bloodshame, during the Middle Ages, was based on the faith of the church. There, sexual intercourse between those who are related with each other until the fourth generation was considered as "bloodshame" in Sweden, with severe penalties. During the 16th century, the death penalty was introduced for "bloodshame", and this penalty was not abolished until 1864 in Sweden. Many countries still have harsh penalities for incest, using various definitions of incest.

The oldest and most common non-religious reason for treating incest as a serious crime is to avoid inbreeding, with its increased risk for certain hereditary diseases. An objection against this justification is that incest-taboo often is applied both to blood-relationships and to in-laws, and that incest is defined very differently in different cultures.

In former times, the word "bloodshame" was used instead of incest in order to emphasize the biological relationship between the perpetrator and the victim. It is however insufficient to see incest only as a genetic problem. Today, more often social rather than biological relations govern the rules for the incest taboo. This means that even when a non-biological parent (for example, a stepfather) abuses a child (who is in a direct dependency relationship to him) sexually, this is also considered to be incest .