A closed pocket or pouch of tissue; a cyst may form within any tissue in the body and can be filled with air, fluid, pus, or other material. Cysts within the lung generally are air filled, while cysts involving the lymph system or kidneys are fluid filled. Cysts under the skin are benign, extremely common, movable lumps. These may develop as a result of infection, clogging of sebaceous glands, developmental abnormalities or around foreign bodies.