Menses is a formal, mostly doctor-and-textbook word for the blood that leaves the body during a period. Friends just say period. The English word comes from Latin mensis, “month,” because the cycle is about a month long; menses was the Latin plural, “months.” The English word already looks plural. Some people say “her menses are late”; some treat it like a stretch of time, “during menses.” Both happen. It is not a rude word, but it is clinic language, not kitchen language. Though actually, menses names the bleeding itself. The wider process — the days, the cramps, the tiredness — is menstruation. And the tidy school sentence “women have menses” is tidier than life: not every woman bleeds every month, and the cycle is not a perfect clock. The word has nothing to do with men, men’s, or Mensa. That is only a look-alike.

經血、月經（醫學／書面；口語多說生理期）