Niddah (or nidah, nidda, nida; ) is a Hebrew term which literally means separation, and generally refers to separation from ritual impurity;Jewish Encyclopedia, Red Heifer The term niddah is overwhelmingly used in Judaism to refer to the rules of Jewish law concerning menstruation. In common usage, a woman is said to be a niddah when she's menstruating, or has menstruated without yet completing the associated ritual requirements. Meaning and use of the term The term niddah appears in the Hebr… (More on Niddah)