A ganglion cyst is a fluid-filled capsule that can cause swelling on ... When the fluid, called synovial fluid, leaks out from these spaces, it can form a sack-like structure called a ganglion ...

The Baker’s cyst or popliteal cyst is also known as a synovial cyst. It is a soft, fluid-filled lump in the popliteal fossa behind the knee. In the back of the knee, there is synovial tissue ...