Pilonidal cyst. Also called jeep drivers disease. This is a cyst that occurs along the buttocks. It was noted during wwii to occur more frequently with people driving more in jeeps. The cyst gets infected and requires removal by surgery. It can recur if you don't get all of the tissue out.
Pilonidal sinus. Pilonidal sinus is a canal that empties into a pilonidal cyst that is the visible portion of the lesion located in the tailbone/sacral area. The name originated from the word "pilus" that stands for hair in greek. It was thought to be an ingrown hair that caused this cyst to develop, but this theory has been refuted over the year. Some people are prone to develop this problem while others are not.