Neuraminic acid is a naturally occurring molecule found in many biological systems. It is known by several different names, including sialic acid, N-acetylneuraminic acid, and KDN (2-keto-3-deoxy-d-glycero-d-galacto-nononic acid). These terms are often used interchangeably, but each refers to a specific type of neuraminic acid molecule. Sialic acid is the most commonly used synonym for neuraminic acid, and it is often found as a component of glycoproteins, glycolipids, and other complex carbohydrates in mammals, birds, and fish. N-acetylneuraminic acid and KDN are less commonly found in these systems but are still important for various biological processes.