Niddah

In biblical terms, a niddah is a woman experiencing a menstrual flow who is thus required to abstain from intimacy with her husband and who renders impure people and items that are susceptible to ritual impurity. After the destruction of the Second Temple, the latter concern fell away, and the term came to refer to a category of Jewish ritual purity law that governs the intimate relations between spouses, also sometimes known as the laws of family purity.