A cyst is a closed pocket or pouch of tissue. It can be filled with air, fluid, pus, or other material.

Cysts may form within any tissue in the body. Cysts within the lung generally are air-filled, while cysts involving the lymph system or kidneys are fluid-filled. Certain parasites, such as trichinosis, dog tapeworm (Toxocara canis), and echinococcus, can form cysts within the muscles, liver, brain, lungs, and eyes.

Cysts are common on the skin. They develop as a result of infection, clogging of sebaceous glands, or around foreign materials.

See also: Pilonidal dimple