In common usage, the eyelid crease is often meant to describe a natural inward creasing of skin seen in the upper eyelid, typically dividing it into a lower segment adjacent to the upper eyelashes, and an upper segment of skin that starts from the crease to the border of the eyebrow.
The crease is present in about 50% of the Asian population, and found to be more prevalent in all non-Asians. The presence of a crease in an Asian is what differentiates a double eyelid (with a crease unequally dividing the lid into two sections, hence “double”) from an Asian with single eyelid or mono-lid.
Double eyelid crease surgery is a form of surgery to add or supplement an eyelid crease to an individual who seeks it. This is often for an individual who does not have a crease, or find that their crease is insufficient, or unbalanced between the two sides.
The reasons for electing this type of cosmetic enhancement may be plenty, and no individual tend to have the same reasons. The commonly cited and reasonable indications will include a desire to have the eyelid opening (fissure) more apparent, since a single eyelid without crease often has a small fold of skin overhanging the opening and makes it look covered over and smaller than it actually is, making it narrower in vertical as well as perceived horizontal dimensions.
Other reasons may be to enhance the ability to apply make-up without smudging, to save time, to correct asymmetry, to create consistency and constancy, or to fulfill their correct impression that having a crease that simulates a natural crease makes the eye more attractive. The important phrase here is natural. It meant that the single-lidded individual almost always want the crease to mimic the dimensions of an Asian double eyelid.