Keritot, Chapter Two, Mishnah Five
Our mishnah defines what type of female is referred to in Leviticus 19:20-22.
To which type of female slave [does this refer]?
1) To one who is half a slave and half a free person, as it is written: And she has been redeemed and not redeemed (Leviticus 19:20), the words of Rabbi Akiva.
2) Rabbi Ishmael says: to a full female slave.
3) Rabbi Eliezer ben Yaakov says: all other forbidden sexual relations are stated explicitly, and of the remainder there is only one who can be half a slave and half a free person.
Section one: According to Rabbi Akiva, the type of female slave referred to in the verse is one who is half a slave and half free (for instance she was owned by two masters and one freed her). He derives this from the strange phrasing in the Torah. The simple meaning of these words is that she has not been redeemed, but the Hebrew syntax allows Rabbi Akiva to interpret the words as if she has been partially redeemed/freed.
Section two: Rabbi Ishmael interprets more in accordance with the simple meaningthe female slave referred to is fully a slave. She has not been redeemed at all.
Section three: All of the other prohibited relations in the Torah are fully free females (they could not be related to the man if they were slaves). There is, according to Rabbi Eliezer ben Yaakov, a long standing tradition that there is one prohibited woman who is half slave/half free. The only one that this can be is the female slave referred to in Leviticus 19.
It is interesting that Rabbi Akiva and Rabbi Eliezer ben Yaakov agree that the slave is half free, but they disagree with regard to how we know this; Rabbi Akiva derives it through a midrashic reading of the verse whereas Rabbi Eliezer ben Yaakov derives it from a received tradition. These are two of the main sources of Jewish lawtradition and midrash/interpretation.