Inbreeding is when the parents of a child, an animal, or a plant are closely related — siblings, cousins, or animals from the same small family line. Breeders sometimes do this on purpose to keep a look or a trait, but it also raises the chance of illness passing down, which is why biologists warn about it. With farm animals and plants it is a normal technical word. With people it is a sensitive subject, so handle it with care. Though actually, you will also hear it used loosely about a closed group that never lets new people or new ideas in — intellectual inbreeding — which is a metaphor borrowed from the farm meaning, not a tidy extra sense of its own.