Ingo means followings.
A title of Daijo Tenno (the Retired Emperor)
A posthumous title of the Emperor
A title of Nyoin (woman bestowed with the title "in")
A title conferred on the Imperial Family for good treatment
A title of a temple
A title which was used as a Kaimyo (a posthumous Buddhist name) for the Sekke (line of regents and advisers), Ashikaga Shogun family, Tokugawa Shogun family, Daimyo family (feudal lord family) and other samurai, and the people
Indengo, Jidengo, and Kengo are related to Ingo.
Ingo as an honorific title of the Retired Emperor
In 823, the title Ingo came to be known as the different name of Daijo Tenno when Emperor Saga moved the Imperial Palace to Saga, Kyoto and was called Sagain. Similarly, Emperor Reizei and Emperor Enyu were called Reizeiin and Enyuin respectively after they left their thrones. At first, Ingo indicated an emperor who left his throne while he was alive, but it became a custom to confer Ingo as a posthumous title on the Emperor who passed away during his reign.
Sometimes, there was more than one Retired Emperor or Cloistered Emperor. In that situation, they were discriminated from by being called their residence's name (sometimes it became their posthumous title after their death) or Ichiin (or Honin), Chuin (when there were more than three) or Shinin in their reign order.
Ingo as an honorific title of Nyoin
The Empress Dowager, "FUJIWARA no Senshi", who was Emperor Ichijo's mother, got a title of Nyoin as Higashi Sanjoin, which started a custom that Ingo was conferred on three Empresses: the Empress, the Empress Dowager, and the Grand Empress Dowager, in the Imperial Court. The court hearing held by Kugyo (the court nobles) for giving Ingo was named Ingo-sadame. After FUJIWARA no Senshi, conferring the honorific title Moningo became a general rule, including Jotomonin (FUJIWARA no Shoshi). Conferring Ingo on an empress was called Ingo Senge.
Ingo for good treatment of the Imperial Family
As an example of the former case, Imperial Prince Atsuakira received Ingo as Koichijoin.
There was a rule that the Imperial Prince who did not ascend to the throne was given Ingo as the father of the Emperor when his son was enthroned. (See Daijo Tenno)
Ingo as a title of a temple
After the Heian period, temples where members of Imperial Family entered priesthood were allowed to have the Ingo.
Ingo as Kaimyo
Ingo, which was originally conferred on only the Emperor and three Empresses: the Empress, the Empress Dowager and the Grand Empress Dowager, began being conferred among subjects when the chief adviser to the Emperor, FUJIWARA no Kaneie called himself as Hokoin.
From the Heian period to the Kamakura period, Ingo was used as Kaimyo of the Emperor and the Imperial Family at first, and then Sekke and Shogun families as well. However, as time passed, the use of Ingo as a Kaimyo for Japanese feudal lords, their lawful wives and concubines, and even their vassals became increasingly common. Moreover, after the Muromachi period, Indengo and Jidengo came into being as higher rank titles, and Ingo as Kaimyo became the second rank.
Today, Ingo is appended to Kaimyo and Homyo (a posthumous Buddhist name in Jodo Shinshu [the True Pure Land Sect of Buddhism]) of lay believers.
It came to be recognized as standard that Kaimyo consists of Ingo and Igo (for example, Koji, Daishi, Shinshi and Shinnyo). Especially, the name combined Ingo and Igo is commonly called Inkoji and Inshinshi.
Some sects, such as Jodo Shinshu do not use Igo.