Definition of VITAMIN C. : a water-soluble vitamin C6H8O6 found in plants and especially in fruits and leafy vegetables or made synthetically and used in the prevention and treatment of scurvy and as an antioxidant for foods —called also ascorbic acid.See vitamin C defined for kids.ADVERTISEMENT. a water-soluble vitamin C6H8O6 found in plants and especially in fruits and leafy vegetables or made synthetically and used in the prevention and treatment of scurvy and as an antioxidant for foods —called also ascorbic acid. See vitamin C defined for kids. ADVERTISEMENT.