An ovarian cyst is a fluid-filled sac that develops in an ovary. Most ovarian cysts are benign and cause no symptoms, but some can cause pain, irregular bleeding, or abdominal swelling. Treatment depends on factors like the cyst's size and appearance on ultrasound, as well as whether symptoms are present. For small, asymptomatic cysts, observation with follow-up ultrasounds may be sufficient, while larger or symptomatic cysts may require surgical removal.