Complex sentence: Menstruation is the term used to describe periodic discharge of blood and mucosal tissue from the inner lining of the uterus through the vagina seen in females of certain mammalian species (including humans).
Split into simpler sentences: Menstruation is the term used to describe periodic discharge of blood and mucosal tissue from the inner lining of the uterus through the vagina. This cyclic discharge is seen in females of certain mammalian species (including humans).