What Is a Pilonidal Cyst?
A pilonidal cyst is a fluid-filled sac under the skin in the lower back, near the crease of the buttocks. Some aren't visible, while others can look like a small pit or dimple in the skin.
They don't usually cause problems or need treatment unless they get infected. A pilonidal (pie-luh-NIE-dul) cyst that's infected is called a pilonidal abscess.