Definition of Pilonidal sinus

Pilonidal sinus: A sinus tract from which there may be chronic drainage of pus, due to an embedded tuft of hair, most often in the crease between the buttocks, but can occur elsewhere in the body, as between the fingers in barbers, hairdressers, and animal groomers. Surgical removal of the hair and the inflammatory tissue cures the condition.

The word "pilonidal" is derived from the Latin words pilus (meaning hair) and nidus (meaning nest). Called also a piliferous cyst, pilonidal cyst, or pilonidal fistula.