月經（英語：menstruation, period, monthly，古代叫做癸水、天癸，俗稱姨媽到、好朋友、大姨妈等），是指女性血液或黏膜定期從子宮內膜經陰道排出體外的現象。
The discharge of blood mixed with pieces of cellular tissue from the uterus out through the vagina, which occurs roughly every month in uninseminated women; menstrual flow.