Picture of Pilonidal Cyst
A pilonidal cyst is an abscess that forms above the crease of the buttocks. These initially small areas of infection may become a large, inflamed nodule over time. The cyst may make it difficult or uncomfortable to sit. The infection often begins in a hair follicle. Prolonged sitting may trigger the formation of this type of cyst.
Text Reference: Diseases of the Colon and Rectum, vol. 43, 2000: "Pilonidal Cyst: Cause and Treatment"