Heard the term arachnoid cyst- info please. ?

Spinal fluid balloon: Arachnoid is a membrane in the nervous system which is in contact with spinal fluid. Sometimes the fluid is entrapped in folds of the arachnoid causing a pocket of fluid to form either in skull or spinal canal. At times the cyst can enlarge causing brain or spinal cord compression. Arachnoid cysts can be present at birth or develop later.