DefinitionAn ovarian cyst is a sac filled with fluid that forms on or inside of an ovary.This article is about cysts that form during your monthly menstrual cycle, called functional cysts. Functional cysts are not the same as cysts caused by cancer or other diseases.For more information about other causes of cysts on or near the ovaries, see also:Dermoid cystEctopic pregnancyEndometriosisOvarian cancerPolycystic ovary syndromeAlternative NamesPhysiologic ovarian cysts; Functional ovarian cysts; Corpus luteum cysts; Follicular cystsCauses, incidence, and risk factorsEach month during your menstrual cycle, a follicle (where the egg is developing) grows on your ovary. Most months, an egg is released from this follicle (called ovulation). If the follicle fails to break open and release an egg,