Menstrual is the describing word for things that belong to a period: menstrual blood, menstrual pain, menstrual products, the menstrual cycle. The cycle is the whole roughly-monthly pattern — not only the bleeding days, but the build-up and the shedding. When people say menstrual cup or menstrual pad, they mean tools for those days. It comes from Latin for “monthly,” from mēnsis, “month.” Older notes also point toward moon, because a month was first a moon’s span. In everyday English this is a neutral, slightly medical adjective. You will see it on packets and in health class. There was once a noun menstrual as well, meaning the bleeding itself, but that use is old; what students meet now is the describing word. Pair it with a noun: menstrual cycle, not “a menstrual.” Though actually, the tidy phrase “twenty-eight-day menstrual cycle” is a sketch, not a rule. Real cycles wander, especially when someone is young, stressed, or playing a lot of sport.