An ovarian cyst is a fluid-filled sac on or within an ovary. The great majority are functional cysts — a normal part of ovulation — and are harmless, often causing no symptoms and resolving on their own over a few cycles.
Other types include dermoid cysts and endometriomas (linked to endometriosis). Most need only monitoring, but sudden, severe, one-sided pelvic pain — especially with nausea, vomiting or fainting — can mean a ruptured cyst or ovarian torsion and is a medical emergency. An ovarian cyst is not the same as PCOS. See ovarian cysts.