Pilonidal is an adjective that literally means "nest of hair". It is generally employed in cases of a pilonidal cyst or sinus, signifying a small mass located under the skin, usually localized in the buttock crease that aligns the coccyx. These masses essentially contain hair and skin and tend to be afflicted with recurring infections. They are entirely benign cysts, but when inflamed can cause severe pain. A pilonidal cyst can be surgically removed if it is inflamed, painful, or too large, but the ablation does not guarantee there will be no recurrence.