- a scleroprotein or albuminoid substance, found in the dead outer skin layer, and in horn, hair, feathers, hoofs, nails, claws, bills, etc.
- a fibrous protein that occurs in the outer layer of the skin and in hair, nails, feathers, hooves, etc
- Any of a group of scleroproteins or albuminoids that contain large amounts of sulfur and are the chief structural constituents of hair, nails, and other horny tissues.
- Any of a class of tough, fibrous proteins that are the main structural component of hair, nails, horns, feathers, and hooves. Keratins are rich in sulfur-containing amino acids, especially cysteine. Individual keratin molecules are entwined helically around each other in long filaments, which are cross-linked by bonds between sulfur atoms on different chains. The twining and cross-linking produce strength and toughness.