single eyelid; monolid; eyelid without a crease
Refers to an upper eyelid that lacks a visible fold or crease, common in East Asian eyes. Often contrasted with 二重まぶた (double eyelid).
See also: 二重まぶた (ふたえまぶた)
She has monolids, but she is very charming.
People with single eyelids are said to have difficulty with eye makeup.