According to Jewish law, a father has the right to “sell” his daughter to a man as an amah ivriyah - a Jewish servant girl (See Shemot 21:7). From the Torah it appears clear that the purpose of such an arrangement is for the “master” to have his son (or himself) marry the girl, since the Torah concludes that if such a marriage does not take place, ve-yatzah hinam, ein kasef – she goes free without any payment being made.
In explanation of this passage, the baraita teaches that ve-yatzah hinam, ein kasef means that she will not need to pay in order to be released either after she becomes a na’arah or when she becomes a bogeret. The amora’im discuss why both statements are needed – after all, if she is released at age 12, how can there be a need to release her again at age twelve and a half? Abaye explains that this refers to the case of an aylonit – a woman who never develops to female physical maturity, who skips the stage of being a na’arah.
From the detailed discussions in the Gemara – mainly in Massekhet Yevamot – it appears that an aylonit suffers from a genetic defect that does not allow her to have children. This is a different categorization than an akarah – a barren woman – whose physical and sexual development is ordinarily normal, but cannot have children because of some other deficiency or impediment. From those descriptions it appears that an aylonit can be recognized by certain unique physical traits, including a lack of secondary sex characteristics like pubic hairs. Furthermore, it appears from the Gemara that there are different types of aylonit, ranging from women who have an overabundance of male hormones to those who suffer from Turner syndrome, where only one X chromosome is present and fully functioning. Approximately 98% of all fetuses with Turner syndrome spontaneously abort; the incidence of Turner syndrome in live female births is believed to be about 1 in 2500.
Within Jewish law there are many discussions about the status of an aylonit, mainly because of the lack of secondary female sex characteristics and because they develop at a relatively advanced age. Thus we find questions about when an aylonit is considered to have reached the age of adulthood, which halakha ordinarily defines as physical maturity.