The term 'BODY SAC' is composed of two words, 'CYST' and 'BURSA'. A CYST is a closed sac-like structure that can be found in the body and is filled with fluid or semisolid material. A BURSA is a small fluid-filled sac located between tissues such as muscles, tendons, and skin, which helps reduce friction between them. Together, these two terms refer to a sac-like structure found in the body that is filled with fluid or semisolid material and helps reduce friction between tissues.