A chalazion is a round, red, usually painful swelling in the upper or lower eyelid caused by blockage of one of the meibomian glands (oil-secreting glands that lubricate the eyelids).
Chalazions are also known as meibomian cysts. They resemble styes but, unlike styes, do not develop on the edges of the eyelids.
Warm compresses can be used to ease discomfort. Most chalazions disappear on their own within a few weeks