A chalazion is a chronic inflammation (not infection) of an eyelid oil gland that stimulates the formation of a surrounding cyst. A chalazion may appear suddenly (and be confused with a stye) or slowly over many days. While a chalazion may shrink over several weeks and become painless, its core frequently remains intact. Sooner of later, the cyst flares up again and again and may gradually deform the eyelid and distort the eyelashes.

Styes are caused by a bacterial infection. Usually the bacteria grow in the root (follicle) of an eyelash. An internal hordeolum is caused by infection in one of the tiny oil glands inside the eyelid. A chalazion develops when an oil gland in the eyelid becomes blocked. If an internal hordeolum doesn't drain and heal, it can turn into a chalazion.

Doctors diagnose a stye or chalazion by closely examining the eyelid. It may be hard to tell the difference between a stye and a chalazion. If there is a hard lump inside the eyelid, the doctor will probably diagnose it as a chalazion.

The term chalazion (pronounced chah la' ze on) is derived from the Greek word meaning small lump. It refers to a cystic swelling with chronic inflammation in an eyelid. A gradual enlargement can be felt near the margin of the lid due to the swelling in one of the eyelid oil glands (meibomian).

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