A sty (sometimes spelled stye) is a tender, painful red bump located at the base of an eyelash or under or inside the eyelid. The medical term for a sty is hordeolum (plural, hordeola).
A sty results from an acute infection of the oil glands of the eyelid (meibomian glands) that occurs after these glands have become clogged. A sty also may arise from an infected hair follicle at the base of an eyelash. The bacterium Staphylococcus aureus is responsible for 90%-95% of cases of styes. Staph aureus is frequently found on the skin. A sty can develop as a complication of blepharitis (inflammation of the eyelid).
The term external hordeolum has been used to refer to a sty that develops at the base of an eyelash involving a hair follicle of the eyelid, whereas the term internal hordeolum refers to a sty arising due to an inflamed meibomian gland under the eyelid
A sty is sometimes confused with a chalazion, which is a cyst or a specific type of scarring (due to chronic inflammation) arising in the meibomian glands of the eyelid. In contrast to a sty, a chalazion is usually painless