Amenorrhea means no menstruation, and that’s normal in females before puberty, during pregnancy and lactation, and after menopause.
Now, for menstruation to happen, a female must normal anatomy and a functioning hypothalamo-pituitary-ovarian axis.
In other words, the hypothalamus must secrete gonadotropin releasing hormone, or GnRH, to stimulate pituitary production of FSH and LH.
And, finally, the ovarian follicles must secrete estrogen and progesterone in response to FSH and LH stimulation.
In childhood and early adolescence, estrogen leads to the development of secondary sex characteristics - like the development of breasts, and the appearance of axillary and pubic hair.
And eventually, usually 2 to 3 years after the breasts have started developing, the first menstruation, or the menarche, occurs.
Menstruation marks the beginning of the menstrual cycle, and that cycle repeats every 28 days on average from puberty until menopause. Ovulation happens on day 14 of this average 28 day cycle.
If the oocyte is fertilized by a sperm, then the remains of the follicle keep secreting estrogen and progesterone to help with pregnancy until the placenta develops, and no menstrual bleeding occurs.
In the absence of pregnancy, estrogen and progesterone levels gradually decrease, and when they get really low, on day 28, this leads to menstruation, and the beginning of a new cycle.
So physiology aside, primary amenorrhea is when a female has not begun breast development by age 13 or hasn’t had menarche by age 15.