Ganglion Cyst: A ganglion cyst is a swelling or tumor on a joint or tendon sheath. Ganglionic cysts form on the elbow, knee, foot, shoulder, wrist, toe or finger. Symptoms include a mass that changes in size, is somewhat painful, and the pain is made worse by joint motion. Though many ganglion cysts disappear on their own, cysts may be treated by aspirating the cyst's contents or surgical removal.