sty, in medicine, acute localized infection of one or more of the glands of the eyelid, with pain, swelling, and redness of the lid margin, usually caused by a staphylococcus infection. An external sty usually releases its pus and disappears in a day or so. Hot or cold compresses and antibiotic ointments are used to treat sties. Recurring sties are usually due to uncorrected refractive errors, poor general health, or infection elsewhere in the body. If a sty does not disappear in a few days, a physician should be consulted.