A Baker's cyst is a tense fluid-filled swelling at the back of the knee.
This area at the back of the knee is called the 'popliteal space', so a Baker's cyst is also called a 'popliteal cyst'. The fluid forms in a lubricating pocket (or 'bursa') that is present in every knee - the gastrocnemius/semimembranosus bursa. This lubricating pocket communicates with the main joint cavity and is lined by the same lubricating cells (synovial cells).
If the knee joint is irritated, for example by osteoarthritis or a torn meniscus, the lining of the joint secretes more joint fluid and the knee may swell. If the joint fluid is under pressure some may pass from the main joint cavity into the bursa. Sometimes the entrance to the bursa may have a valve-like quality, allowing fluid into the bursa from the main joint cavity, but not allowing it to escape again. So a little lump appears at the back of the knee towards the inner aspect.