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 obstructed, the oil builds up, leading to swelling and the formation of a chalazion. Chalazia can occur on either the upper or lower eyelid and may resolve on their own over time. Treatment options include warm compresses to help unblock the gland or, in some cases, medical procedures to drain the lump. They are generally not serious but can be bothersome.