Baker's cyst--Popliteal cyst.

Baker's cyst is a swelling that can develop behind the knee. It is filled with synovial fluid which is the lubricating fluid that is usually found inside the knee joint. It most commonly occurs if there is an underlying problem with your knee such as osteoarthritis

A Baker's cyst is a fluid-filled swelling that can develop behind the knee. It is named after a doctor called William Baker who first described this condition in 1877. It is also sometimes called a popliteal cyst, as the medical term for the area behind the knee is the popliteal fossa.

Symptoms can include pain, swelling and tightness behind the knee. Rarely, a Baker's cyst can rupture (split open) and cause similar symptoms to a deep vein thrombosis. A Baker's cyst often gets better and disappears by itself over time. However, there are various treatments that may help if you do have symptoms associated with it.