Junbo means equating a woman who is not the real mother of the emperor with his mother or the title of a woman who is equated with the mother of the emperor.
What is Junbo?
It became the first example of junbo that in 1087, in the succession of Emperor Horikawa to the throne, his elder sister, Imperial Princess Teishi was equated with his mother because his real mother, Chugu (Empress) FUJIWARA no Kenshi was already deceased, after that, junbo was named in the case that an infant emperor succeeded to the throne after his real mother passed away, or the case that although his real mother was alive, her social status was too low or she was already a nyoin (a woman bestowed with the title "in" [title given to retired emperor]).
In selection of junbo, it was selected from the lawful wife of the former emperor who was not the father of the emperor and unmarried imperial princesses who were the elder sisters or the aunts of the emperor. At first junbo was the status which was named accordingly in court ceremony, later it was put to give good treatment to imperial princesses, and there were many examples that imperial princesses who were not married to the emperor were named as a junbo and empress (it is called junbo-rikko [investiture of junbo and empress], and an imperial princess who became an empress in this way is also called a Sonsho kogo [honorific empress who is not a wife of the emperor]).
Although junbo was selected from imperial family, there were some exceptions that TAIRA no Seishi, a daughter of TAIRA no Kiyomori, was named as the junbo of Emperor Takakura who was a Kiyomori's nephew-in-law in the end of the Heian Period, and Yasuko HINO, the lawful wife of Yoshimitsu ASHIKAGA, the 3rd Seitaishogun, was named as the junbo of Emperor Gokomatsu in the Muromachi Period.