a closed sac that develops abnormally in some body structure
"The doctor removed the painful cyst from his cheek after it grew too large under the skin."
A pouch or sac without opening, usually membranous and containing morbid matter, which develops in one of the natural cavities or in the substance of an organ.
"The doctor diagnosed a benign cyst on my kidney after noticing it had grown during the routine ultrasound."