A chalazion is a bump on the eyelid caused by inflammation of the oil glands. It often starts out as a stye, but unlike a stye, it is not usually painful. The bump may get better on its own or need medical treatment. Without treatment, the bump may remain for months or years.
Swollen bump on the upper or lower eyelid that may be red
Usually painless and firm to the touch
May form a head like a pimple and drain pus
An ophthalmologist (eye doctor) can diagnose a chalazion using a biological microscope.