hyaluronic acid, naturally occurring polysaccharide found in the extracellular matrix of vertebrate tissues, particularly soft connective tissues. It is also found in certain body fluids, including fluid in the eyes and the synovial fluid of joints.

Hyaluronic acid has several important functions, among them a protective role, wherein it serves as a scavenger of harmful free radicals and other chemical agents in the extracellular environment. In joints, where it is produced by synovial lining cells, hyaluronic acid is an essential lubricant.

Hyaluronic acid is approved for certain medical uses. For example, injection of hyaluronic acid during cataract surgery reduces the risk of postsurgical dry eye disease. In osteoarthritis, hyaluronic acid may be used to help relieve pain and joint stiffness. Hyaluronic acid is also found in some over-the-counter dietary supplements and cosmetic products, particularly creams and moisturizers.