A red tender bump on the eyelid that is caused by staphylococcal bacteria.
According to WebMD, stys may be confused with chalazia, which are also lumps in the eyelid but are caused by meibomian gland blockage.
Stys may heal on their own, or may be treated with warm compresses, antibiotic ointments or surgical drainage.
WebMD
St. Luke's Eye
MedicineNet.com on stys