sebaceous cyst is a closed sac or cyst below the surface of the skin that has a lining that resembles the uppermost part of a hair follicle and fills with a fatty white, semi-solid material called...
a cyst on the sebaceous gland or the skin.
I have them; mine are (fortunately for me) almost all on the scalp hidden by hair). In practice, they are a lot like pimples that never entirely go away....