Monolid
An upper eyelid with no visible crease, so the lid looks smooth from lashes to brow.
Also: single eyelid
A monolid is an upper eyelid without a visible crease. It is common among people of East, Southeast and Central Asia, and is also found among Indigenous peoples of the Americas, some peoples of Africa and people of many other backgrounds. Some people have one monolid and one creased lid. Makeup artists work with monolids using gradient shadow and liner thick enough to show when the eye is open. It is a normal eyelid form.