Final answer:
A pilonidal cyst is a cyst that commonly develops in the skin at the base of the spine. It is an encapsulated sac located just below the upper layers of skin.
Step-by-step explanation:
A cyst that commonly develops in the skin at the base of the spine is called a pilonidal cyst. It is an encapsulated sac filled with fluid, semi-solid matter, or gas, typically located just below the upper layers of skin. Pilonidal cysts usually occur in the sacrococcygeal area, which is the lower part of the spine near the tailbone.