Epidermoid cysts (epidermal cyst, epidermal inclusion cysts, keratin cysts) are frequently incorrectly called sebaceous cysts. One of the most common benign skin tumors in adults, epidermoid cysts are rare in childhood and infancy. These semisolid cysts are lined by a keratinizing epithelium and filled mostly with macerated keratin, which has a cheese-like consistency and pungent odor. They frequently appear to arise spontaneously. Alternatively, they may result from disruption of follicular structures or by implantation of the epidermis via a penetrating injury.

Epidermoid cysts are a feature of several hereditary syndromes, such as Gardner syndrome; cysts are usually multiple and in unusual locations in Gardner syndrome.