A cyst is a closed sac, having a distinct membrane and division COMPARED with the nearby tissue. Hence, it is a cluster of cells that has grouped together to form a sac (not unlike the manner in which water MOLECULES group together, forming 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. It may contain air, fluids, or semi-solid material. A A cyst is a closed sac, having a distinct membrane and division compared with the nearby tissue. Hence, it is a cluster of cells that has grouped together to form a sac (not unlike the manner in which water molecules group together, forming 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. It may contain air, fluids, or semi-solid material.