Chalazion: A cyst of the little glands in the eyelids that make a lubricant which they discharge through tiny openings in the edges of the lids. The lubricant is a fatty substance called sebum characteristic of sebaceous glands.
These glands are called the meibomian glands. Inflammation of them is termed meibomianitis or, alternatively, meibomitis. Chronic inflammation of the meibomian glands leads to meibomian cysts or chalazions.
The meibomian glands are named for a 17th-century German anatomist Heinrich Meibom (who must have had good eyes to see these minute structures). They are also known as the palpebral glands, tarsal glands, or tarsoconjunctival glands.