Form of vitamin C; other forms include ascorbic acid, L-ascorbic acid, ascorbyl palmitate, and magnesium ascorbyl phosphate. Calcium ascorbate, often referred to as Ester-C, is considered a stable form of vitamin C and an antioxidant.
References for this information:
Medical Science Monitor, October 2007, issue 10, pages 205–210
Journal of Cosmetic Science, November–December 2006, issue 6, pages 465–473