A cyst is a closed sac, having a distinct envelope and division compared with the nearby tissue. Hence, it is a cluster of cells that has grouped together to form a sac (like the manner in which water molecules group together to form a bubble); however, the distinguishing aspect of a cyst is that the cells forming the "shell" of such a sac are distinctly abnormal (in both appearance and behaviour) when compared with all surrounding cells for that given location. A cyst may contain air, fluids, or semi-solid material. A collection of pus is called an abscess, not a cyst. Once formed, a cyst may resolve on its own. When a cyst fails to resolve, it may need to be removed surgically, but that would depend upon its type and location.
Cancer-related cysts are formed as a defense mechanism for the body following the development of mutations that lead to an uncontrolled cellular division. Once that mutation has occurred, the affected cells divide incessantly and become cancerous, forming a tumour. The body encapsulates those cells to try to prevent them from continuing their division and contain the tumour, which becomes known as a cyst. That said, the cancerous cells still may mutate further and gain the ability to form their own blood vessels, from which they receive nourishment before being contained. Once that happens, the capsule becomes useless, and the tumour may advance from benign to cancerous.
Adrenal cyst: Types of adrenal cysts include parasitic cysts, epithelial cysts, endothelial cysts, and pseudocysts. 56% of all adrenal cyst-like changes are pseudocysts, and only 7% of those pseudocysts are malignant or potentially malignant.
This is just one example of how the Greek root cyst-, which simply means a fluid-filled sac, also is found in medical terms that relate to the urinary bladder and the gallbladder, neither of which involve cysts.
^Zadik, Yehuda; Akta? Alper; Drucker Scott; Nitzan W Dorrit (2012). "Aneurysmal bone cyst of mandibular condyle: A case report and review of the literature". J Craniomaxillofac Surg. 40 (8): e243-8. doi:10.1016/j.jcms.2011.10.026. PMID22118925.