An epidermal inclusion cyst, also known as an epidermoid cyst, is a benign growth that develops in the skin. It occurs when epidermal cells, which form the outermost layer of the skin, become trapped and multiply within a sac-like structure. These cysts are typically filled with keratin, a protein found in skin, hair, and nails, and can appear anywhere on the body but are most commonly found on the face, neck, and trunk.