An ovarian cyst is a fluid-filled sac (follicle) that contains a maturing egg. The cyst forms on the ovary's surface, then disappears not long after an egg is released to travel down the fallopian tube. If a mature egg is not released or if the sac reseals after releasing the egg, the sac can swell up with fluid, forming a functional ovarian cyst.

Most of these cysts do not cause symptoms and they go away by themselves. Some cause mild pain. Others can cause severe pain when they rupture or bleed.