A chalazion is a swelling in the eyelid that creates a cyst. It is caused by inflammation of one of the small oil-producing glands (meibomian glands) in the upper and lower eyelids. The word chalazion comes from the Greek for a small lump.
A chalazion is not the same as a stye, which is also a lump in the eyelid. A stye is an infection of a lash follicle that forms a red, sore lump near the edge of the eyelid, while a chalazion is an inflammatory reaction to trapped oil secretions.
Chalazion are not caused by bacteria, although they can, rarely, become infected by bacteria. Antibiotics therefore do not play much role in their treatment. In most cases, antibiotics are more likely to cause side-effects than to help.
Chalazion tend to be situated further from the edge of the eyelid than styes, and are generally pointed towards the inside of the eyelid. Sometimes a chalazion can cause the entire eyelid to swell suddenly.
When a chalazion is small and without symptoms, it may disappear on its own. If a chalazion is large, it may cause blurred vision.