What is a stye?

A stye is indeed characterized as a swollen bump on the edge of an eyelid. This condition occurs when the glands at the base of the eyelashes become infected, often leading to a localized infection that presents as a painful, red lump. The underlying cause is typically a bacterial infection, most commonly from Staphylococcus bacteria, which can lead to inflammation and swelling.

While the other options relate to various eye conditions or symptoms, they do not accurately define what a stye is. The first option describes a bacterial infection of the cornea, which is known as keratitis and is distinct from the localized infection that constitutes a stye. The third option refers to general eye irritation, which could encompass a wide range of issues but does not specifically identify the characteristic swelling of a stye. The fourth option mentions an eye condition linked to aging, which is more relevant to conditions such as cataracts or macular degeneration, highlighting additional key differences from a stye.