Elastin: A protein that coil and recoils like a spring within the elastic fibers of connective tissue and accounts for the elasticity of structures such the skin, blood vessels, heart, lungs, intestines, tendons, and ligaments. 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. Also called elasticin.