pilonidal cyst [L, pilus + nidus, nest] , a cyst that often develops in the sacral region of the skin. Pilonidal cysts may sometimes be recognized at birth by a depression, sometimes by a hairy dimple in the midline of the back in the sacrococcygeal area. Usually these cysts do not cause any problems, but occasionally a sinus or fistula develops in early adulthood that communicates with the skin, resulting in infection. A fistula also may develop to the spinal tract from a pilonidal cyst. If a cyst becomes infected or inflamed, it is excised, and the space is surgically closed after the infection or inflammation has been effectively treated. A pilonidal cyst is not a true cyst but a poorly drained anaerobic abscess.
