A ganglion cyst is a swollen, closed sac under the skin.
The sac is attached to the sheath of a tendon or may be attached to a joint. The cyst contains fluid similar to the fluid that is in your joints. It can vary in size from a small pea to a golf ball. Ganglion cysts most often occur on the wrist, at the end joint of a finger, or at the base of a finger. They also occur on top of the foot.
The cause of ganglion cysts is not known. You may feel discomfort or pain. Sometimes the area of the cyst becomes swollen or disfigured.