A chalazion is a painless, slow-growing lump that develops within the eyelid as a result of a blocked and inflamed meibomian gland — one of the many tiny oil glands that line the edges of the eyelids. When the gland's opening becomes obstructed, secreted oil accumulates and the surrounding tissue becomes inflamed, forming a firm, round nodule beneath the skin of the eyelid. Chalazia most commonly appear on the upper eyelid but can also affect the lower lid.
It is important to distinguish a chalazion from a stye (hordeolum): a stye is an acute bacterial infection of a gland at the eyelid margin, causing sudden pain, redness, and swelling, whereas a chalazion is a chronic, non-infectious granulomatous inflammation that is typically painless. Small chalazia often resolve on their own with warm compresses, while larger or persistent ones may require a steroid injection or minor surgical drainage.

