1. The essential constituent of yellow elastic connective tissue which is brittle when dry, but when moist is flexible and elastic.
2. An extracellular connective tissue protein that is the principal component of elastic fibers in the middle layer of arteries.

Elastin functions in connective tissue together with collagen. Whereas elastin provides elasticity, collagen provides rigidity to connective tissue. Elastin is normally no longer made after puberty and aging begins.