Pilonidal sinus is a medical condition that involves the development of a small hole or tunnel in the skin, usually in the area between the buttocks. It is commonly referred to as a "pilonidal cyst." The condition can be painful and uncomfortable and can lead to infection if left untreated.

The cause of pilonidal sinus is not completely understood, but it is believed to be related to hair follicles and friction in the affected area. The condition is more common in men than women, and it typically develops in young adults.

Symptoms of pilonidal sinus may include pain, swelling, and redness in the affected area. There may also be drainage of pus or blood from the sinus. In severe cases, fever and malaise may develop.

Treatment for pilonidal sinus typically involves surgical removal of the sinus. This may involve simple excision of the sinus, or more extensive surgery to remove surrounding tissue and close the wound. Antibiotics may be prescribed to treat any infection that is present.

Preventive measures include maintaining good hygiene in the affected area, avoiding prolonged sitting, and keeping the area dry and clean. In some cases, hair removal or laser hair removal may also be recommended.