The breeding of
people/ animals/ plants that are closely related to each other (first cousins or even closer), thus keeping the genepool
weak and leaving them vulnerable to various health problems, mentally and emotionally as
well as physically.
Purebred
dogs and royalty are notorious for inbreeding; to keep the line "pure" then they are only allowed to procreate with their "own kind", even when offspring are stupid, sickly, deranged,
ugly or any number of things.
Carlos II of Spain was the end result of the Spanish Hapsburgs; his
parents were uncle and niece and both were highly inbred. Carlos was deformed,
retarded, delicate, and had an extended infancy and premature senility. He was married twice, but he was impotent and sex was beyond his abilities.
Plants can be inbred, too; bananas of today are very different from those of three generations ago and often lack the genetic diversity which would keep them healthy.