Jun Daijo tenno (准太上天皇)
Jun Daijo tenno is the treatment which is equal to that of Daijo tenno (retired emperor).
Summary
There was an example that a member of imperial family who didn't ascend the throne received the treatment equivalent to that of Daijo tenno because his son ascended the throne while he was still alive.
People often misunderstand as if such a concrete status and a title had existed because it is written in "Genji Monogatari" (The Tale of Genji) that Hikaru Genji, the main character of the story, took "the position which was equal to Daijo tenno", however, Jun Daijo tenno is neither a position nor a title but treatment. The noun of "Jun Daijo tenno" doesn't exist at all, and moreover, it is correct to read it as a sentence, such as "Daijo tenno ni junzu."or "Daijo tenno ni nazurau." (Both of them mean that "to equate with Daijo tenno").
Thinking about conferment of Ingo (title given to retired emperor), entitlements of nenkan (the right to apply for government posts) and nensyaku (the right to apply for official court ranks; nenkan and nensyaku were part of the income of the aristocracy), and the right to set up In no cho (office of retired emperor), it can be said that in a sense, nyoin (a woman bestowed with the title "in" [similar to Ingo; title given to retired emperor]) is also the same position equivalent to Joko (the abbreviation of Daijo tenno). In the decision of the treatment of Higashisanjoin FUJIWARA no Senshi who became the first nyoin in 991, the treatment of Joko was taken into account to decide her treatment because it was an unprecedented procedure and it was necessary to rank her, who was the real mother of Emperor Ichijo but inferior in authority to the Empress due to lack of the career as an empress, above the Empress, and that became the precedent of the treatment of following nyoins. However, because the requirements to get the status of nyoin, which was originally the good treatment for real mother of emperor, gradually diversified as time passed, not all nyoins were able to receive the same treatment as that of Joko.