The onset of menstruation in a young girl is called menarche.
Menstruation
Menstruation is the process of the shedding of the uterine lining on a regular monthly basis in women.
Uterus prepares itself every month to receive a fertilized egg. If fertilization has not taken place then the uterus, or womb, sheds its lining along with unfertilized egg, blood, and mucus. This process is called menstruation.
Menarche
The onset of menstruation in a young girl is called menarche.
In a girl, it is the beginning of the menstrual cycle when she attains puberty at the age of 10 to 15 years.