Menstruation is the name of the process: each month the lining of the womb is shed, and blood leaves the body. Textbooks and clinics use this word. Friends more often say period. It is not dirty and not a joke; it is just the formal name. It is built from menstruum + -ation, after a late Latin word. So the stem is the old “monthly” word, and -ation makes a noun for a process. That is why it feels like a school-and-clinic word. You can count it: a first menstruation, painful menstruation. You can also speak of menstruation as a general fact of life. Though actually, people stretch it a little: “during menstruation” can mean the whole time of the period, not only the mechanism inside. It does not mean the same as menopause (when periods stop for good, usually later in life). And it is not a word for mood: you can be tired or sore during menstruation, but menstruation itself is the bleeding process. Cycles wander. The old idea that menstruation is dirty is a social story, not a body fact.