If you develop a lump on your eyelid, it could be either a “stye” or a “chalazion.” A stye occurs when an oil or sweat gland in the eyelid (usually in an eyelid follicle) becomes infected with bacteria (i.e., Staphylococcus aurus). Characterized by a small yellowish spot inside an inflamed red pimple, styes occur on the outside of the eyelid and are more painful than chalazia. A chalazion occurs when a “meibomian” or other sebaceous (oil) gland in the eyelid becomes blocked and triggers an immune response. The result is a firm, roundish, and (usually) painless lump, which more commonly forms on the upper eyelid. Never squeeze a stye or a chalazion. Medical treatment may be required.
P.S. A stye is often professionally treated by pulling out the affected eyelash(es) to allow for drainage. Chalazia may be treated with corticosteroid injections, incision, or surgical removal.