Noun
1. A family of fibrous structural proteins. Keratin is the key structural material making up the outer layer of human skin. It is also the key structural component of hair and nails. (wikipedia.org)
2. A fibrous scleroprotein that occurs in the outer layer of the skin and in horny tissues such as hair, feathers, nails, and hooves. (wordnetweb.princeton.edu)
Word origin: From Greek keras (genitive keratos) “horn” + chemical suffix -in usually indicating a neutral substance, antibiotic, vitamin, or hormone