Why is a menstrual period called a period?

Answer 1

It's a shorter way of saying "menstrual period" as opposed to other kinds of periods, such as orbital periods, periods of time, etc. The etymology of the word (Greek) is something along the lines of "Interval, season, time, circuit" etc.

Answer 2

The term "period" likely originates from Latin meaning "orbit" or "cycle." It refers to the monthly cycle of shedding the uterine lining, which is a recurring event. The term has been used historically to describe this regular process in a woman's reproductive system.