Shikken

The was a titular post held by a member of the Hōjō clan, officially a regent of the shogunate, from 1199 to 1333, during the Kamakura period, and so he was head of the "bakufu" (shogunate). It was part of the era referred to as .
During roughly the first half of that period, the "shikken" was the "de facto" military dictator of Japan (not including the independent Northern Fujiwara). The title of "shikken" was modified, as second in command to the "Tokusō" in 1256, but by the Muromachi period (1333–1573) the position, though not abolished, did not even figure in the top ranks. The position ceased to exist after the Muromachi period.
Etymology.
The word "shikken" is the on'yomi reading of the combination of the two kanji characters and , each meaning "to hold (something in the hand, or a service or ceremony); to administer", "power, authority" respectively. Therefore the word literally means "to hold power/authority".
"Shikken" as supreme ruler (1199–1256).
Though officially a regent for the "shōgun" in the Kamakura shogunate in Japan, on paper a "shikken" derived power from the "shōgun", in reality the actual "shōgun" had been reduced to a figurehead in a similar marginalizing manner just as the emperor and imperial court earlier had been reduced to figureheads by the "shōgun". Both the posts of "shikken" and "tokusō" were monopolized by the Hōjō clan.
Hōjō Tokimasa, who was the father-in-law of the first "shōgun" Minamoto no Yoritomo, father of Hōjō Masako, became the first "shikken" in 1203, after Yoritomo's death. The "shikken" was the chief of the "mandokoro" at that time. Tokimasa became the de facto ruler of the shogunate by monopolizing decisions for the young "shōguns" Minamoto no Yoriie and Minamoto no Sanetomo (Yoritomo's sons and Tokimasa's own grandsons), executing whoever got in his way, family or not. Tokimasa's own grandson (Yoriie) and great-grandson were murdered on Tokimasa's orders, a year after he replaced Yoriie (the second "shōgun") with Sanetomo.
Tokimasa's son Yoshitoki strengthened the post of "shikken" by integrating it with the post of chief of Samurai-dokoro, after annihilating the powerful Wada clan, who had dominated the latter position. The "shikken" became the highest post, controlling puppet "shōguns" in practice. In 1224, Yoshitoki's son Hōjō Yasutoki set up the position of "rensho" (cosigner), or assistant regent.
"Shikken" as "tokusō" subordinate (1256–1333).
Hōjō Tokiyori separated the two posts of "tokusō" (initially head of the Hōjō clan) and "shikken" in 1256. 
He installed Hōjō Nagatoki as "shikken" while designating his son Tokimune to succeed as "tokusō". Effective power was moved from "shikken" to "tokusō". Tokimune, contemporaneous with Mongol invasions of Japan, at one point personally occupied all 3 most powerful posts of the shogunate, and thus Japan: "tokusō", "shikken", and "rensho".