Final answer:
Menarche is the first menstrual period that marks the beginning of menstruation in females during puberty.
Step-by-step explanation:
One of the most significant changes in females during puberty is menarche, which is the first menstrual period. It marks the beginning of menstruation. In the United States, menarche occurs at an average age of 12.4 years. However, there is considerable variation in this age, with menarche at any age between eight and 16 considered normal.