Pilonidal Sinus
A Pilonidal sinus also called Pilonidal cyst, Pilonidal abcess or Sacrococcygeal fistula is a cyst (small sac) or tunnel in the skin. It develops over the tailbone at the top of the cleft of the buttocks. The cyst usually contains hair and skin debris. More than one cyst may develop and these are linked by tunnels under the skin.