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Wiktionary
hyaluronic acid

n. (context organic compound English) A mucopolysaccharide that forms a gel-like substance within animal connective tissue

WordNet
hyaluronic acid

n. a viscous mucopolysaccharide found in the connective tissue space and the synovial fluid of movable joints and the humors of the eye; a cementing and protective substance

Wikipedia
Hyaluronic acid

Hyaluronic acid (HA; conjugate base hyaluronate), also called hyaluronan, is an anionic, nonsulfated glycosaminoglycan distributed widely throughout connective, epithelial, and neural tissues. It is unique among glycosaminoglycans in that it is nonsulfated, forms in the plasma membrane instead of the Golgi, and can be very large, with its molecular weight often reaching the millions. One of the chief components of the extracellular matrix, hyaluronan contributes significantly to cell proliferation and migration, and may also be involved in the progression of some malignant tumors.

The average 70 kg (154 lb) person has roughly 15 grams of hyaluronan in the body, one-third of which is turned over (degraded and synthesized) every day. Hyaluronic acid is also a component of the group A streptococcal extracellular capsule, and is believed to play a role in virulence.