Give the biological/technical terms for the following:
The onset of menstruation in a young girl.

Explanation

**Step-by-Step Solution:**

1. **Understanding Menstruation**: Menstruation is a biological process that involves the shedding of the endometrium (the lining of the uterus) if fertilization does not occur. This process happens periodically, typically every month.

2. **Identifying the Trigger**: Menstruation is initiated when an egg released from the ovaries is not fertilized. The body prepares for a potential pregnancy each month, and if it does not happen, the endometrium is shed.

3. **Recognizing the Age of Onset**: The onset of menstruation marks a significant milestone in a young girl’s development, indicating that she has reached puberty.

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