"To define the eyes" means to enhance their appearance by using makeup or techniques that make them stand out and look more noticeable, such as using eyeliner, mascara, or eyeshadow. Full definition
Style your hair into a fish - tail braid and define your eyes with a stroke of vibrant eye liner to get noticed. (myntra.com)
If you have trouble creating defined eye looks (or it feels like you are just mixing a bunch of colors together on your eye) try smaller brushes like this one. (justalittleblush.com)
Always use dark brown eye pencil for defining eye brows or use eye pencil according to the color of your eye brows. (latestasianfashions.com)