Commentary for Niddah 1:3

ארבע נשים דיין שעתן – that the don’t regularly see blood.

Rabbi Eliezer says: there are four types of women for whom it suffices [to reckon] their [period of uncleanness from] the time [of their discovering the flow]. A virgin, A pregnant woman, A nursing woman; And an old woman. According to Rabbi Eliezer, there are four types of women who, since it is not common for them to menstruate, need not be concerned lest their menstrual flow started earlier than they discovered it. They defile only the things they touch after they discovered that they were bleeding. The following mishnayot will elaborate more on each of these women.

אבל הלכה כרבי אליעזר – and this is what comes out from the Gemara (Tractate Niddah 7b) that the Halakha is according to Rabbi Eliezer.

Rabbi Joshua says: I have only heard [the ruling applied to] a virgin, but the halakhah is in agreement with Rabbi Eliezer. Rabbi Joshua initially seems to adopt a traditional approach here. He restricts the halakhah to the tradition that he received this law applies only to a virgin. We should note that it is usually Rabbi Eliezer who acts as a traditionalist. However, if the last sentence of the mishnah is also the words of Rabbi Joshua, then he too admits that even though he only heard a limited ruling, the halakhah is in accord with the expanded version put forth by Rabbi Eliezer. We might surmise that if the original tradition referred only to a virgin (as R. Joshua claims), the expanded version (which includes all four categories) reasoned that if it is sufficient to reckon the uncleanness of a virgin from the time of discovery because she does not regularly menstruate, the same would be true of other women who don't regularly menstruate. And if you are surprised that a "virgin" doesn't regularly menstruate, the rabbinic definition will be in tomorrow's mishnah. Stay tuned.