Keratin is a strong and durable protein [a substance your body uses to build itself] found in the outer layer of our skin, hair, nails, and in the feathers, hooves, and horns of animals. For example, when you touch your hair or nails, you are actually touching keratin.
Very Simple Definition:
Keratin is a tough substance found in our hair and nails. For example, our hair is made of keratin.