A sebaceous cyst /sɪˈbeɪʃəs sɪst/ is a term commonly used to refer to either:
Epidermoid cysts (also termed epidermal cysts, infundibular cyst), or
Pilar cysts (also termed trichelemmal cysts, isthmus-catagen cysts).
Both of the above types of cyst contain keratin, not sebum, and neither originates from sebaceous glands. Epidermoid cysts originate in the epidermis and pilar cysts originate from hair follicles. Therefore, technically speaking they are not sebaceous cysts. “True” sebaceous cysts, cysts which originate from sebaceous glands and which contain sebum, are relatively rare and are known as steatocystoma simplex or, if multiple, as steatocystoma multiplex.
It has been suggested by medical professionals that the term sebaceous cyst be avoided since it can be misleading.:31 In practice however, the term is still often used for epidermoid and pilar cysts.