Definition of Onah
The word onah literally means "time period." In the context of the laws of niddah, it usually refers to a day or a night. Each 24-hour day thus consists of two onot. The daytime onah begins at sunrise (henetz hachamah, commonly called netz) and ends at sunset (shekiat hachamah or shekiah). The night-time onah lasts from sunset until sunrise.
Marital relations are forbidden on an onah (as defined above) when a woman anticipates her menses. This is called an onat perishah, "time period of separation."
The term onah can also refer to the length of the menstrual cycle. Halachically, we assume that the onah beinonit, or "average interval," is thirty days long.
Finally, the term mitzvat onah (a mitzvah performed at a set time period) refers to a husband's conjugal obligations toward his wife and is also used as a halachic euphemism for marital relations.
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