Infection of an eyelid gland. An external sty results from infection of a sebaceous gland at the edge of the eyelid; tears flow and the eye feels tender, as if something is in it. The sty reddens and swells. Warm compresses help it break sooner. An internal sty is caused by infection of a meibomian gland under the eyelid lining. More painful than an external sty, it usually breaks through the inner lining of the lid when it discharges and may leave a painless cyst (chalazion) at the site. See alsoboil.