Definition: In the female reproductive system, a follicle is a fluid-filled sac that contains an immature egg, or oocyte. Follicles are found in the ovaries. During ovulation, a mature egg is released from the follicle. While several follicles begin to develop each cycle, normally only one will ovulate an egg. The follicles that do not release a mature egg disintegrate. The follicle that releases an egg turns into a corpus luteum.