What is the name of the cosmetic used to define the lash line?

The cosmetic used to define the lash line is eyeliner.

Eyeliner is applied along or around the contours of the eyes to make the lash line look darker, thicker, or more sharply shaped. It can be drawn above the upper lashes, beneath the lower lashes, or along the waterline. Winged eyeliner extends beyond the outer corner, while tightlining places product very close to the lashes to create a fuller-looking lash base.

The practice has ancient roots. People in ancient Egypt, Mesopotamia, and India used dark eye products, including kohl made with mineral pigments such as galena and antimony. Historical uses were not purely decorative: eyeliner was also associated with beliefs about protection and, in some accounts, shielding the eyes from bright desert conditions.

Eyeliner is different from mascara, which coats and darkens the eyelashes themselves. Eyeshadow adds color across the eyelid, while an eyebrow pencil is intended for brow hairs and shape. Modern eyeliner comes in pencil, liquid, gel, powder, cake, and felt-tip forms, each producing a different degree of softness or precision.