Final answer:
Menarche is the term used to describe a girl's first menstruation or first menstrual period. It marks the beginning of menstruation and is a significant event in a girl's life. The average age of menarche in girls in the United States is around 12.5 years.
Step-by-step explanation:
Menarche is the term used to describe a girl's first menstruation or first menstrual period. It marks the beginning of menstruation and is a significant event in a girl's life as it signifies the beginning of her reproductive capability. The average age of menarche in girls in the United States is around 12.5 years, although there can be variation due to genetic and environmental factors.