A pilonidal cyst is an abnormal pocket in the skin that usually contains hair, skin debris and other abnormal tissue. A pilonidal cyst is almost always located near the tailbone at the top of the cleft of the buttocks. The term "pilonidal" comes from the Latin words for hair ("pilus") and nest ("nidus").
Pilonidal cysts occur more often in men than in women, and they are most common in people between the ages of 15 and 24.
A pilonidal cyst may remain harmless. If it becomes infected, however, it will cause pain, inflammation and usually drainage of fluids.