Lip liner is a cosmetic product used to define and shape the lips. It typically comes in pencil, liquid, or retractable forms and is available in various colors to match or complement lipstick shades. Applying lip liner helps prevent lipstick from feathering or smudging, ensuring a clean and polished look. To use lip liner, one usually outlines the natural lip line before applying lipstick. Some people also fill in their lips with lip liner for added color and longevity. This versatile product can enhance the overall appearance of the lips and is a staple in many makeup routines.