In Japanese History, Kampaku ("white barrier") was the title given to a regent ruling in place of a sitting adult emperor.
As with other nations, the Japanese government had long had a system whereby a regent could be appointed to rule in cases where a an emperor ascended to the throne while still a young child. This regent was called a Sesshō (摂政), was required to resign once the emperor came of age, and had traditionally been required to be another member of the Imperial Family, usually an uncle or somesuch.
However, by the 9th century, the powerful Fujiwara family of nobles had come to dominate the position of Sessho, from which they effectively ruled Japan in place of a succession of powerless child emperors under their control.
But on occasion, do to a temporary lack of a suitable heir, the Fujiwara were obliged to allow an emperor to reach adulthood, which meant they had to give up the title of Sessho and simply hope that the emperor would continue to heed their advice. This was obviously a rather intolerable situation for the power-hungry Fujiwara, so in 880, the then clan head of the Fujiwara, Fujiwara Mototsune, invented the position of Kampaku, or regent for an adult emperor, to allow the Fujiwara to continue ruling even after an emperor reached adulthood.
Over time, a tradition evolved that only direct descendents of the Fujiwara could hold the title of Kampaku, and the office continued to be duitifully filled by a Fujiwara descendant long after the Fujiwara had lost power and the Shogun held all real political authority.
One of the most famous people to later hold the title of Kampaku was the warlord Toyotomi Hideyoshi. Once Hideyoshi had taken over Japan, he wanted to have a suitably old and grandiose title to justify and legitimate his rule. The best title to call himself would have been "Shogun," but tradition demanded that only direct descendents of the Imperial Family could become Shogun.
In this situation most warlords would simply have quickly forged some documents allegedly proving their descent from a branch line of the Imperial Family, such as the Minamoto clan. However, Hideyoshi's background was so humble that to claim he was an Imperial decendent would have strained credulity beyond all acceptable bounds (legend held that he was the son of a woodcutter). However, saying he was a descendent of the Fujiwara was vaguely more plausible, so he had himself named Kampaku instead of Shogun.
The word for a retired Kampaku was Taikō (太閤), and even in modern Japanese Hideyoshi is still often known simply as "Taikō", since he used that title for much of his later rule, having passed on the title of Kampaku to his son, Hidetsugu.