If a woman has a flow of blood outside the context of her period… (Leviticus 15:25)
Sometimes you get equal time when you’d rather not. This is probably one of those times. Mitzvos #178-179 deal with the zav, which is a man who has an emission from the male organ that is not semen. This mitzvah discusses the analogous female condition.
The Torah says that if a woman has a flow of several days outside of the context of her regular menstrual period, she is what’s called a zavah (basically the female equivalent of a zav). The Talmud (Niddah 72a-b) clarifies that there are 11 days in between periods during which a woman’s irregular flow would render her a zavah. If she sees blood once or twice during this time, the woman is called a “zavah k’tanah” (a minor zavah); she can wait one day and be purified. However, if she sees blood on three consecutive days during this time, she is what’s called a “zavah g’dolah” (a major zavah) and must wait an entire week before being cleansed.
As with niddah impurity, she is ritually unclean and capable of transmitting this status to others. Also like niddah, she must wait seven days before immersing in a mikvah in order to be purified. Unlike a niddah, but like a zav, she must bring a sacrifice upon becoming ritually cleansed. (This sacrifice is the topic of the next mitzvah.)
The zavah is kind of a hybrid mitzvah. In some ways she’s like a niddah and in some ways she’s like a zav. The underlying rationale of this mitzvah would seem more likely to approach that of the niddah than that of the zav since the zav’s status was potentially brought on by his own overindulgence. Here, the woman does not appear to have done anything that might have contributed to her status as a zavah (although she does bring a sacrifice, as we will discuss). The Sefer HaChinuch appears to attribute the symptoms of a zavah as a sign of a physical malady that might be harmful to others. In any event, a zavah might be well-advised to use her period of impurity for personal reflection, the same as the zav, the metzora and others so affected.
This mitzvah applies in all times and places to both men and women. (Remember, men can also be rendered unclean by women in such conditions, so they must likewise observe laws relating to this form of impurity.) As with other forms of ritual impurity, it has no real application in the absence of the Temple. This mitzvah is the discussed in the Talmudic tractate of Niddah; see, for example, the section starting on page 35a. It is codified in the Shulchan Aruch in Yoreh De’ah 196 and is #106 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos.