A dominant follicle is the largest follicle that is produced in each menstrual cycle of a woman and that will trigger ovulation. Therefore, the dominant follicle is the one that ends up maturing and expelling the egg that it contains inside.
In a woman's natural cycle, multiple ovarian follicles develop, but only one will reach the final stage and will be the dominant one. This follicle will have a size of 22-24 mm approximately. The rest of the follicles that started to develop will degenerate and disappear through a biological process.