A chalazion is a lump that slowly forms due to blockage and swelling of an oil gland in the eyelid. A chalazion is generally not an infection. It often starts out as a very small red, tender, swollen area of the eyelid. In a few days, it may change to a painless slow-growing lump the size of a pea.
A chalazion is often confused with a stye, which is an infection of an oil gland in the eyelid. A stye produces a red, swollen, painful lump on the edge or the inside of the eyelid and usually occurs closer to the surface of the eyelid. If untreated, a stye can result in the formation of a chalazion.