Niddah, Chapter 1, Mishnah 6. The theme running through this chapter is tumah that reaches backwards in time, and the chapter set out a group of four women to whom the rule of dayan sha'atan applies, "their hour suffices for them": should blood appear, the tumah begins at that instant and nothing earlier is affected. The group consists of a besulah, meaning a girl who has not yet experienced blood at any point in her life; a me'uberes, once her pregnancy can be recognized; a meinikah, a mother nursing her child; and a zekeinah, a woman advanced in years. Our mishnah supplies the definition of the last of the four, after which two Tanna'im come to widen the rule.
Eizo hi zekeinah
The mishnah poses its question and answers it. "Eizo hi zekeinah?" Which woman falls under the heading of zekeinah? "Kol she'avru aleha", any woman for whom there have already elapsed "shalosh onos samuch le'ziknasah", three onos at a point in life bordering on her old age.
The measure is three onos, meaning three full monthly periods that have gone by without her seeing blood, at a stage of life when she is already an older woman. Once an older woman has missed her period for three consecutive months, she may assume that she has reached menopause and that periods are no longer expected of her. Accordingly, if she does see blood, it renders her temei'ah only prospectively, from that point onward, and not retroactively.
Rebbi Eliezer: any woman who missed three onos
"Rebbi Eliezer omer", Rebbi Eliezer takes age out of the equation. "Kol ishah", any woman whatsoever, including one still young in years, "she'avru aleha shalosh onos", if three onos have gone by for her without blood, is "daya sha'atah". As far as this halacha is concerned, she stands on the same footing as the zekeinah described by the mishnah.
The Tanna Kamma does not accept this. According to him the leniency belongs only to a woman whose years make menopause a reasonable assumption; a younger woman who happens to have missed three months does not receive it. Rebbi Eliezer holds that the three missed onos are decisive on their own, even in a younger woman.
Rebbi Yosi: three onos for the me'uberes and the meinikah as well
"Rebbi Yosi omer: me'uberes u'meinikah", regarding a pregnant woman and a nursing woman Rebbi Yosi reopens the question of when their status begins. "She'avru aleha shalosh onos", only after three onos have gone by for them, are they "dayan sha'atan". The mishnah before ours had ruled that pregnancy confers the status as soon as it can be recognized, and that nursing confers it for as long as the mother nurses. Rebbi Yosi requires more: in each case one starts the count from when the pregnancy becomes recognizable, three onos must pass with no blood, and the leniency takes effect only from then on.
The halacha
Rebbi Yosi is thus arguing with the Tanna Kamma of the previous mishnah, and the halacha follows that Tanna Kamma rather than Rebbi Yosi. Nor do we rule like Rebbi Eliezer of our mishnah. The zekeinah who is daya sha'atah is an older woman over whom three onos have passed close to her old age, while the me'uberes and the meinikah receive their status at the times set out for each of them, without waiting for three missed months.