A monolid is an eyelid shape that doesn’t have a crease. An eye that does have a crease is known as a double eyelid.
Monolids are typically a facial feature of Asian people. However, other races may also have a monolid or an eyelid shape without a crease.
In certain regions, especially East Asia, surgery and cosmetic products are commonly used to create the appearance of a crease. In fact, it’s one of the most popular types of plastic surgery in this area of the world.