Kouin (Imperial descendant or posterity) (皇胤)
The term Kouin refers to Imperial descendants.
Summary
Before the Japanese modern history, the term was most commonly used among many synonyms. As it is indicated that the Chinese character '胤(IN)' reads 'Tane' in its Japanese pronunciation, which means paternal blood, the term Kouin was used for only descendants in the male line. This is because in the paternal society, the lineage was defined as male line for granted.
As Razan HAYASHI mentioned that Ieyasu TOKUGAWA was an Imperial descendant (it means Ieyasu was one of Seiwa-Genji or Minamoto clan), generation difference is not considered. The thought of so-called an unbroken line of the Japanese Imperial Family simply means that the Imperial Throne has to be succeeded to the person who is an Imperial descendant.
People who were considered Imperial descendants in the genealogical tree
Empress Jingu The 5th grandchild of the Emperor Kaika
TAKENOUCHI no Sukune The 4th grandchild of the Emperor Kogen
HEGURI no Matori The 6th grandchild of the Emperor Kogen It is said that he tried but failed to become Emperor of Japan.
SOGA no Umako The 10th grandchild of the Emperor Kogen
TACHIBANA no Moroe The 4th grandchild of the Emperor Bidatsu
ARIWARA no Narihira The 2nd grandchild of the Emperor Heizei (Heijo)
TAIRA no Masakado The 5th grandchild of the Emperor Kanmu
Sei Shonagon; Seishonagon The 10th grandchild of the Emperor Tenmu