keratin
/ˈkɛrətɪn/
noun
- A strong protein that forms hair, nails, skin, and the outer layer of horns, hooves, and feathers.
- Shampoos often contain keratin to help strengthen hair.
- Your fingernails are made mostly of keratin, which makes them hard.
- Birds' feathers are rich in keratin, giving them durability and flexibility.