A sac that is filled with fluid that forms on the surface or inside an ovary. During a woman’s menstrual cycle, a follicle forms in her ovary that is the home of an egg that will be released during ovulation. If the follicle does not break open and fails to release an egg, the fluid that remains in the follice forms a cyst. This is usually perfectly normal when it occurs before menopause and it will often go away on its own in 8 to 12 weeks. If a woman gets them frequently, birth control pills can help. To be on the safe side, a woman should check with her healthcare provider if she thinks she might have an ovarian cyst.