follicular atresia

Breakdown of ovarian follicles, which consist of an oocyte surrounded by granulosa cells and internal and external theca cells.

Typically around 20 follicles mature each month but only a single follicle is ovulated and the follicle from which the oocyte was released becomes the corpus luteum. And the rest from which the oocyte was released undergo follicular atresia.
It occurs continually throughout a woman's life, as she is born with millions of follicles but will only ovulate around 400 times in her lifetime.