A cyst from an ingrown hair, medically known as a pilar cyst, is a small, round, fluid-filled sac that forms under the skin. It is caused by an ingrown hair that curls back and grows into the skin instead of out of it, creating a small, tender lump.
Pilar cysts are common and can occur anywhere on the body where hair grows, such as the scalp, face, armpits, and pubic area. They are usually harmless and do not require treatment, but they can sometimes become infected or inflamed, which can cause pain and discomfort.