incest in·cest (ĭn'sěst')
Sexual relations between persons who are so closely related that their marriage is illegal or forbidden by custom.
The statutory crime of sexual relations with such a near relative.
Sexual relations between relatives who are forbidden by law to marry; for example, between father and daughter or mother and son.
Note: Though each society has its own system for determining the range of people who fall into this category, every society has an incest taboo of some sort.