A chalazion is a small, painless lump that forms on the eyelid due to a blocked oil gland. These glands, known as meibomian glands, produce oil that helps keep the eye lubricated. When the gland becomes clogged, it can lead to swelling and the formation of a chalazion, which may vary in size. Chalazia are generally not serious and often resolve on their own within a few weeks. Treatment options include warm compresses to help unblock the gland and, in some cases, medical intervention by an eye care professional. They are different from styes, which are usually painful and caused by bacterial infections.