Incest is sexual activity or marriage between very close family members. It is a taboo, as well as a criminal offense, in virtually all societies. In many societies premarital sex is allowed or encouraged; in most such societies, the same restrictions apply to sexual unions as to marriage.

The precise meaning of incest varies widely, because different cultures have differing notions of "sexual activity" and "close family member." Some jurisdictions consider only those related by birth, others also 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.

Sometimes the word "incestuous" is also used metaphorically to describe other inappropriately close relationships, for example between an authority figure and a subordinate, or between people in the same profession or creative field.

"Incest" and "inbreeding" are not synonymous. Whereas incest is a cultural category in which incestuous and non-incestuous relations are categorically distinct, "inbreeding" is a scientific category that refers to mating between individuals of varying degrees of genetic closeness. In many societies, the definition of incest relations and the degree of inbreeding may correlate positively (for example, any sexual relations between people of a given degree of genetic closeness is considered incestuous). In many other societies, the definition of incest and the degree of inbreeding may not correlate (for example, sexual relations between certain people of a given degree of genetic closeness are considered incestuous, whereas sexual relations between other people of the same degree of genetic closeness are not considered incestuous.)

The consequence of inbreeding is to increase the frequency of homozygotes within a population. Depending on the size of the population and the number of generations in which inbreeding occurs, the increase of homozygotes may have either good or bad effects.