Niddah, Chapter 1, Mishnah 5. The opening mishnayos of this chapter deal with the question of retroactive tumah: when a woman discovers blood, do we assume that it was already present earlier, so that whatever she touched or handled beforehand becomes tamei as well? The Mishnah earlier listed four women about whom we do not make that assumption at all. They are dayan sha'atan, "their moment is enough for them": they become temei'os only from the time they see the blood and onward, with nothing at all reckoned backwards. Those four are a besulah, a me'uberes, a meinikah and a zekeinah. Our mishnah now turns to the first three of them and asks precisely who fits into each category.
Eizo hi besulah
Eizo hi besulah? Whom does the Mishnah classify as a besulah? Its answer: kol shelo ra'asah dam, miyameha, afilu nesu'ah, meaning any female who has not yet experienced bleeding at any point in her life, and this holds true even of a woman who is already married.
Note that the term besulah is functioning here as a technical label, quite apart from its everyday connotation. The Mishnah is not addressing the question of virginity whatsoever. For this halacha the title belongs to any female who has yet to menstruate even a single time, and it stays with her even if her twelfth birthday is long behind her, and even if she has a husband. Since nothing in her past gives us reason to suspect that bleeding preceded her discovery, there is no foundation for a retroactive assumption, and she is therefore daya sha'atah: her tumah begins with the sighting itself and extends to nothing earlier.
Eizo hi me'uberes
"Eizo hi me'uberes? Mishe'yivada ubarah." From when does a pregnant woman receive this lenient status? From the point at which her pregnancy is recognizable, that is, from when it can be seen that she is carrying a child. Until then she is not yet included in the list; from that point on, blood that she sees renders her temei'ah only going forward.
Eizo hi meinikah
"Meinikah, ad she'tigmol es bnah." In the view of the Tanna Kamma of our mishnah, a nursing mother retains this standing for as long as the nursing continues, and it lasts until the weaning of her child.
The Mishnah then presents the case where the nursing comes to an end: "Nasnah bnah le'meinikah, gemalato o meis." She handed her child over to a wet nurse who will do the nursing in her place; or she weaned him, so that he no longer nurses from his mother; or, tragically, the infant died. What is her status now?
"Rebbi Meir omer": in Rebbi Meir's opinion, a woman in any of these three situations may no longer be classified as a meinikah. She falls back under the standard law and once again conveys tumah backwards, metama me'es le'es, across the full twenty four hours that preceded her sighting of blood.
"Vachachamim omrim: daya sha'atah." The Chachamim disagree and rule that she remains daya sha'atah. In their view the status of meinikah is measured not by whether she is actively nursing but by the twenty four months following childbirth. For those twenty four months, whether she is still nursing her child or has stopped for whatever reason, the Chachamim regard her as having the halachic standing of a meinikah. Rebbi Meir ties the status exclusively to the act of nursing itself, so that once she has given the child to a wet nurse, or weaned him, or stopped nursing for any other reason, the leniency falls away.
This is the machlokes between Rebbi Meir and the Chachamim, and the halacha follows the Chachamim: a woman enjoys the status of meinikah, and with it the rule of daya sha'atah, throughout the twenty four months after she gives birth.