Lip liner is a cosmetic product used to define and enhance the shape of the lips. Typically available in pencil, liquid, or gel form, it comes in various shades to match or complement lipstick colors. Lip liner helps prevent lipstick from feathering or bleeding outside the lip line, ensuring a clean and polished look. To apply lip liner, users outline the natural shape of their lips, starting from the center and moving outward. Some people also fill in their lips with lip liner before applying lipstick for added longevity. Popular brands include MAC, Urban Decay, and NYX, each offering a range of colors and formulas.