A pilonidal sinus is a small, abnormal channel or tract in the skin, typically found near the coccyx (tailbone) at the top of the cleft of the buttocks. It often results from the ingrowth of hair, which can cause an infection or abscess. This condition can cause pain, swelling, and drainage of pus or blood.

While the exact cause is not fully understood, contributing factors include hair penetrating the skin, prolonged sitting, and certain physical activities that involve friction or pressure on the area. Treatment options vary depending on the severity and may range from antibiotics and drainage procedures to surgical excision. Pilonidal sinus is more common in young adults and has a higher incidence in men than women.