Inbreeding is a term in genetics, meaning the crossing (mating) of closely related animals or plants. Self-fertilisation in plants is the most extreme kind of inbreeding. It is quite widespread in plants which carry both male and female flowers on the same plant. Inbreeding is the opposite of outcrossing, which is the mating of unrelated members of the same species.
Other terms are inbred strain, a group of organisms so inbred that they have little or no genetic variation, and inbreeding depression, which is the decreased fitness (usually lack of fertility and early illness and death) brought about by inbreeding.
For human beings, inbreeding is a very destructive trait, and almost all cultures in the world have attempted to restrict inbreeding e.g. by prohibiting marriages between first cousins.
It is mating with blood relatives within the nuclear family.
Inbreeding Facts for Kids. Kiddle Encyclopedia.