Niddah (Hebrew: נִדָּה) is a masekhet or tractate of the Mishnah and the Talmud, and is part of the order of Tohorot. The content of the tractate primarily deals with the legal provisions related to Halakha of Niddah. Niddah, along with Eruvin and Yevamot, is considered one of the three most difficult tractates in the Babylonian Talmud. A Hebrew mnemonic for the three is עני (ani, meaning "poverty").