What is a cyst?
A cyst is a closed sac or balloon-like structure that is not a normal part of the tissue where it is found. Cysts are common and can occur anywhere in the body in persons of any age. Cysts usually contain a gaseous, liquid, or semisolid substance. Cysts vary in size; they may be detectable only under a microscope or they can grow so large that they displace normal organs and tissues. The outer wall of a cyst is called the capsule.
WebMD. (2005, December 08). Retrieved from http://www.webmd.com/skin-problems-and-treatments/cysts