Female Emperor (女性天皇)
It means Emperor who was woman. They were often called female Emperor (they were also called Jotei in Goon and Nyotai in Karagoe), after there was active argument of the issue of Imperial succession since 2004, they were commonly called female Emperor in an official document or in the publication.
There were eight female emperors in ten generations in Japanese history. All the female Emperors including Empress Jingu and Iitoyo no himemiko (Princess Iitoyo) were female Imperial members of the male line in the Imperial Family (Female Emperors of the male line), when they were married, they were Emperor's Empresses before succession to the throne and the husbands (Emperors) previously passed away. These six female Emperors in eight generations were all in between the late 6th century and the late 8th century.
Emperor Suiko (her reign was 592 - 628) (the thirty third)
Emperor Kogyoku (her reign was 642 - 645) (the thirty fifth)
Emperor Saimei (her reign was 655 - 661) (the thirty seventh, the second accession to the imperial throne of Emperor Kogyoku)
Emperor Jito (her reign was 686 - 697) (the forty first)
Emperor Genmei (her reign was 707 - 715) (the forty third)
Emperor Gensho (her reign was 715 - 724) (the forty fourth)
Emperor Koken (her reign was 749 - 758) (the forty six)
Emperor Shotoku (her reign was 764 - 770) (the forty eighth, the second accession to the imperial throne of Emperor Koken)
Emperor Meisho (her reign was 1629 - 1643) (the hundred and ninth)
Emperor Gosakuramachi (her reign was 1762 - 1770) (the hundred and seventeenth)
*Before the modern period the Empress Jingu was counted among successive Emperors.
Princess Iitoyo was not officially stated as Emperor as she was called the daughter of Emperor Richu in the Kojiki (Records of Ancient Matters), and was called the daughter of Prince Ichinobe no Oshihano in Nihon shoki (Chronicles of Japan), however she was called 'the twenty fourth Emperor Iitoyo' in Fusoryakki (A Brief History of Japan), and there were some of the opinion that she should be considered as Emperor.
By the way a female emperor simply means female emperor, but sometimes the word, female emperor can be understood as emperor of the female line since it has a similar meaning. However female Emperor means the Imperial members who succeed the Imperial blood line from these maternal relatives, and this does not concern whether the Emperor herself/himself is male or female, (Thus, there are both male and female emperors existing in the female lineage) so female Emperor and Emperor of the female lineage are basically two different words with different concepts.
The movement in relation to female Emperors
According to Clause 1 of the current Imperial House Act, it is said 'the male Imperial member of the Imperial Family in the male lineage should succeed to the throne.'
Not only a female is not allowed to succeed to the throne which there is no example in history, but also female in male lineage is not allowed to succeed to the throne. The way to deny Imperial succession of both a female and the Emperor of the female lineage may have caused the confusion to understand the meaning of two different concepts of these words.
However there were no male Imperial successors born into the family since Imperial Prince Akishinonomiya Fumihito, for forty years, in addition to this, the Crown Prince, Imperial Prince Naruhito's first child was the Toshinomiya Imperial Princess Aiko, there was a movement to revise the Imperial House Act to allow female Emperors or Emperors in female lineage to succeed to the Imperial Throne. (Please refer to the issue of Imperial Succession)
In September 2006, the male Imperial member, Imperial Prince Akishinonomiya Hisahito was born for the first time in forty one years, however the issue of the shortage of male Imperial members was yet unsolved, the Imperial succession problem still remains the same.
Furthermore, the reason why the idea of a female Emperor was not positively accepted was because of the issue in terms of 'Shinto ceremony.'
There are many types of Shinto ceremonies held at the Imperial Palace in which women were not allowed to attend. Even the successive female emperors cancelled that kind of the ceremony.
In the current argument about female Emperors, since the 'Shinto ceremony' was not discussed, there is some criticism that there was no consideration by the Imperial Family who succeeded the ritual.
The role of the female Emperor
Generally the descriptions in the Kojiki (Records of Ancient Matters) and the Nihonshoki (Chronicles of Japan) were respected for the details on female Emperors, all the female Emperors in history were female emperor of the male lineage, there were no examples of female emperors married to non Imperial Family members with her children succeeding to the throne.
Throughout history, there is the generally known theory of 'female Emperor's temporarily succeeding' to avoided the discontinuation of the Imperial lineage (Imperial male line), and to keep the Imperial succession in the male line.