This refers to a persistent swelling at the back of the knee. The swelling is caused by fluid in the knee joint itself which bulges out at the back. It is usually a sign of another problem in the knee.
A definite swelling is felt at the back of the knee which may or may not be mildly painful. It may fluctuate with day to day activities, and be tender to touch. It is often associated with some sort of problem in the knee (a torn meniscus, or damage to the articular cartilage) and this additional problem may cause other symptoms. Sometimes the cyst can burst on its own giving a period of pain in the calf which then resolves.
If you think you have this condition it is well worth getting it checked out by Mr Willis-Owen. In the mean time you should avoid any aggravating activities. It generally won't get better on its own unless it spontaneously ruptures.
Treatment involves first confirming the diagnosis then establishing the underlying cause. An ultrasound scan or MRI scan can be useful to exclude other problems. Treatment options focus on the underlying cause, but may include drawing fluid out of the cyst and injection of steroids. Rarely surgery is needed to address the cyst.