*Istihaadah:It is the blood that comes out of the vagina of the woman other than menstruation or postnatal bleeding. It is an irregular vaginal bleeding. Moustahaadah (i.e. woman that has Istihaadah) is considered legally pure and the rulings of pure women apply to her.
In fact, she has, regarding such blood, three cases:
First:When a woman used to have a stable, regular menstrual period (i.e. having certain duration of menstrual period) before she had Istihaadah. That is to say before Istihaadah, she used to menstruate for five or eight days in the beginning or in the middle of the month. So she knows its number and time. Thus, such a woman can wait until her usual period ends and the rulings on menstruation are applicable in this case. When her period ends, she has to take a ritual bath and perform prayer, then deem any other bleeding as Istihaadah.
Second: When a woman does not have a regular menstrual period, but her bleeding is always distinguishable; sometimes she bleeds malodorous, thick, and black blood (i.e. having the characteristics of menstruation blood), and some other times she bleeds red blood which is neither thick nor malodorous. Such a woman is to consider the former kind of blood as that of her menstrual period and she has to take a ritual bath when it ends. She should regard the other kind as that of Istihaadah, the period in which she can perform prayer and observe fasting, for she is considered ritually pure in this case.
Third:When a woman has neither a regular menstrual period nor a distinguishable kind of blood (e.g. the confused woman). Such a woman must follow the prevalent womanly menstrual period (i.e. average of six or seven days a month) and deem whatever discharge after this period as Istihaadah.