A highlighter is a colored pen or marker used to emphasize or mark important information on a page by adding a bright and noticeable color to it. Full definition
This is the most beautiful of highlighters in my book! (justinasgems.com)
To keep this dramatic shade from looking too heavy, Black suggests pairing it with curled lashes, subtle contouring, and a touch of highlighter on the inner corners of the eyes, like Nars Soft Touch Shadow Pencil in Goddess. (allure.com)