A molecule your skin makes naturally that holds large amounts of water.
Evidence & sources
Hyaluronic acid is a natural part of skin and helps it hold water; its amount in skin declines with age. Evidence: moderate. Known: Laboratory and physiological research. Unknown: Evidence that topical products reverse skin aging is limited. Source: Papakonstantinou et al., Dermato-Endocrinology, Hyaluronic acid: A key molecule in skin aging (2012).